EXCHANGE 


H 


MAE    2    1913 

;TUDIES  OF  FLINT  CLAYS  AND  THEIR  ASSOCIATES 


A  THESIS 

PRESENTED  TO  THE  FACULTY  OF  THE  GRADUATE  SCHOOL 

OF  CORNELL  UNIVERSITY  FOR  THE  DEGREE  OF 

DOCTOR  OF  PHILOSOPHY 


BY 


SYDNEY  LONGMAN  GALPIN 


Reprint  from  Transactions  American  Ceramic  Society,  Vol.  XIV,  P.  301,  1912 


REPRINTED  FROM  THE  TRANSACTIONS  OF  THE  AMERICAN  CERAMIC  SOCIETY.          VOL.  XIV. 
[Paper  read  at  Chicago,  111.,  Meeting,  March,  1912.] 


STUDIES  OF  FLINT  CLAYS  AND  THEIR  ASSOCIATES. 

By  SYDNEY  L.  GALPIN,  Ithaca,  N.  Y. 
INTRODUCTION. 

Non-plastic  fire  clays  have  been  recognized  for  many  years 
as  a  type  differing  distinctly  from  the  plastic  fire  clays,  yet  the 
investigation  of  their  nature  and  origin  has  received  but  little 
attention.  Their  geologic  occurrence  and  distribution  in  Mary- 
land,1 Pennsylvania,2  Ohio,3  Kentucky,4  West  Virginia,5  Mis- 
souri6 and  Washington7  have  been  briefly  described  by  different 
authors;  and  they  are  also  known  to  occur  west  of  Pueblo  in 
Colorado.8 

Greaves- Walker9  discusses  the  sedimentation  of  the  Ken- 
tucky flint  clays  and  concludes  that  they  were  deposited  in 
basins  of  rather  limited  area.  Hopkins10  has  argued  from  a 
study  of  some  of  the  Pennsylvania  fire  clays  that  they  were  once 
the  soil  in  which  the  flora  of  the  coal  beds  flourished  and  that 
they  owe  their  purity  largely  to  the  leaching  action  of  the  plant 
roots.11  He  also  points  out  an  explanation  of  flinty  nature  similar 
to  Wheeler. 

Wheeler12  has  described  in  detail  the  flint  clays  of  Missouri. 
He  considers  them  to  be  sediments  deposited  in  limestone  sinks 
and  purified  by  the  leaching  of  carbonated  meteoric  waters. 
Their  non-plastic  quality  may  be  due  to  crystallization  of  kaolin- 


1  Md.  Geol.  Surv.,  Report  on  Allegheny  County;  Ries,  Clays  of  Maryland,  Md.  Geol. 
Surv.,  Vol.  IV,  Pt.  III. 

2  Hopkins,  Clays  of  Western  Pennsylvania,  Ann.   Rept.   Penn.  State  College,   1897; 
Woolsey,  Clays  of  Ohio  Valley  in  Pennsylvania,  Bull.  225,  U.  S<    Geol.  Surv. ;  Lines,  Clays, 
and  Shales  of  Clarion  Quadrangle,  Bull.  315,  U.  S.  Geol.  Surv.;  Shaw  and  Munn,  Foxburg 
Quadrangle,  Bull.  454,  U.  S.  Geol.  Surv. 

3  Newberry,  Geology  of  Ohio,  Vol.  Ill;  E.  Orton,  Jr.,  Geol.  Surv.  of  Ohio,  Vol.  VII, 
Part  1. 

4  Greaves-Walker,  Flint  Fireclay  Deposits  of    N.    E.    Ky.,    Trans.    A.    C.    S.,  IX, 
1907;  W.  C.  Phalen,  Clay  Resources  of  N.  E.  Ky.,  Bull.  285,  U.  S.  Geol.  Surv. 

5  Grimsley  and  Grant,  W.  Va.  Geol.  Surv.,  Vol.  Ill,  1905. 
0  Wheeler,  Clays  of  Missouri,  Vol.  XI,  Mo.  Geol.  Surv. 

7  Shedd,  Clays  and  Clay  Industry  of  Washington,  June,  1910. 

8  Personal  communication,  H.  Ries. 

9  Loc.  cit. 

10  Loc.  cit. 

11  If  this  theory  is  correct  it  seems  curious  that  many  clays  underlying  coals  are  so  im- 
pure. 

12  Loc.  cit. 


257698 


302  STUDIES  OF  FLINT  CLAYS  AND  THEIR  ASSOCIATES. 

ite,  because  of  their  coarsely  platy  structure  as  seen  under  the 
microscope. 

Others  have  made  preliminary  microscopic  examinations 
which  will  be  discussed  later. 

The  investigations,  upon  which  this  paper  is  based,  were 
undertaken  with  a  view  to  determine  if  possible  the  nature  of 
the  differences  between  plastic  and  non-plastic  fire  clays.  They 
were  carried  on  partly  in  the  field  and  in  part  at  the  geological 
laboratories  of  Cornell  University.13  The  field  work  included 
a  study  of  the  refractory  clays  at  the  following  localities:  Stras- 
burg,  Ohio;  Clarion,  Pa.;  Woodland,  Clearfield  County,  Pa.; 
Mt.  Savage,  Md. ;  and  Olive  Hill,  Ky.  Many  sections  from  these 
localities  were  also  studied  under  the  microscope. 

DETAILED  ACCOUNT  OF  OCCURRENCES. 

In  presenting  the  results  of  this  investigation  it  seems  best 
to  give  the  details  of  the  occurrence,  petrography  and  other 
properties  first;  and  then  to  explain  the  views  or  theories  formu- 
lated as  a  result  of  these  studies. 

STRASBURG,   OHIO. 

The  plastic  and  non-plastic  clays  of  Tuscarawas  County 
are  found  at  several  horizons  in  the  Lower  Productive  Measures 
(Alleghany  series).  Probably  the  most  important  flint  clay 
is  that  worked  about  two  miles  east  from  Strasburg  by  the  National 
Fire  Brick  Company.  The  bed  outcrops  only  near  the  crests 
of  the  higher  hills  and  seems  to  be  at  the  same  horizon  as  New- 
berry's14  No.  7  coal,  which  is  thought  to  be  the  equivalent  of  the 
upper  Freeport  coal. 

An  unusually  good  exposure  exists  in  the  company's  open- 
cut  workings  from  which  the  accompanying  sketch  shown  in 
Fig.  i  is  taken.  The  clay  bed  consists  of  three  divisions,  -viz., 
a  lower,  No  i  grade  fire  clay;  a  middle,  flint  fire  clay;  and  an 
upper,  No.  2  grade  fire  clay. 


13  The  author  is  pleased  to  acknowledge  his  indebtedness  to  Professors  Ries  and  Gill 
for  counsel  and  assistance  in  conducting  the  work,  and  to  the  National  Fire  Brick  Co., 
The  Harbison- Walker  Refractories  Co.,  The  Union  Refractories  Co.,  The  Olive  Hill  Fire 
Brick  Co.,  and  Mr.  Wm.  Ramsay,  of  Mt.  Savage,  Md.,  for  courtesies  and  aid  in  obtaining 
materials. 

14  Geology  of  Ohio,  Vol.  Ill,  p.  66. 


STUDIES  OF  FLINT  CLAYS  AND  THEIR  ASSOCIATES. 


303 


' COARSE  TO.  M EDI U M '•  SAN DSTON  E •  •' .'  :/.'•  V', '' . '. '' 


SHALES   WITH    SIDERITE 
CONCRETIONS  NEAR  BASE. 


COAL  E- 


NO.H  FIRE  CLAY   I  -6  FT. 


SEMI -FLINT  ALONG  CONTACT. 
FLINT  CLAY   0-6  FT 


NO.I  FIRE  CLAY  Z-8FT 
"IRON"  NODULES. 


Fig.    1.— Section  at  Strasburg,  Ohio. 

The  No.  i  fire  clay  is  a  gray  laminated  rock  having  an  ir- 
regular hackly  fracture.  It  shows  some  variation  in  color  and 
texture,  becoming  more  sandy  toward  the  bottom,  where  occasional 
clay-iron  stones  may  be  seen.  It  is  frequently  contorted  and 
always  cut  by  a  network  of  slickensided  slip  planes.  Though 
hard  when  mined  it  becomes  somewhat  plastic  when  ground 
in  a  wet-pan.  Its  thickness  is  rather  variable,  due  to  "rolling 
up"  in  places  and  "squeezing  out"  in  others.  The  clay  rests 
upon  an  even  sandstone  floor  but  its  upper  boundary,  though 
usually  well  defined,  is  irregular. 

The  flint  fire  clay  has  a  massive  appearance  and  is  broken 
up  into  blocks  by  irregular  joints,  but  slip  planes  so  charac- 
teristic of  the  surrounding  clays  are  wanting.  It  breaks  with 
a  clean  conchoidal  fracture,  showing  a  light  buff  surface  mottled 
by  greenish  gray  lenticular  markings,  and  lined  in  an  irregularly 
concentric  manner  by  bluish  bands  or  ribbons.  The  latter  seem 
to  be  generally  grouped  about  bunches  of  small  pyritic  concre- 
tions. Upon  exposure  the  clay  breaks  up  into  angular  fragments 
stained  by  the  decomposition  of  the  pyrite.  Carbonaceous 
material  is  locally  present  in  quantities  sufficient  to  produce  a 
black  clay.  The  thickness  of  the  flint  clay  is  decidedly  variable, 
there  being  frequent  marked  swells  and  pinches.  The  boundary 
between  the  flint  clay  and  the  No.  2  fire  clay  is  indefinite,  the 
term  "semi  flint"  describing  the  transition  clay,  as  it  is  made 


304          STUDIES  OF  FLINT  CLAYS  AND  THEIR  ASSOCIATES. 

up  of  lenses  of  flinty  clay  more  or  less  surrounded  by  the  slicken- 
sided-and  more  plastic  variety. 

The  No.  2  fire  clay  bears  a  close  resemblance  to  the  No. 
i,  but  shows  perhaps  even  greater  contortion  and  variation 
in  thickness.  Its  upper  contact  with  the  base  of  a  three  foot 
coal  bed  is  even  and  regular. 

The  total  thickness  of  the  clay  bed  is  fairly  constant,  varia- 
tions of  one  kind  being  compensated  by  reciprocal  changes  in 
thickness  of  the  others.  The  strata  of  the  region  show  no  signs 
of  regional  disturbance,  indicating  that  the  movements  in  the 
clay  bed  were  of  local  nature. 

MICROSCOPIC    PETROGRAPHY    OF   THE    STRASBURG    CLAYS. 

Section  /.Io — This  represents  the  flint  clay  of  this  locality. 
With  low  magnification  (40-80  dia.)  one  sees  a  fine  more  or  less 
angular  ground  mass  traversed  by  irregular  veins  usually  under 
15  microns  in  width,  which  are  bordered  by  a  brownish  scum 
of  limonite  (?)  and  filled  with  material  similar  to  the  general 
ground  mass.  Numerous  small  pyrite  crystals  lie  in  this  scum 
(see  Plate  I,  Fig.  i)  and  a  few  are  scattered  through  the  clay. 
Spherical,  radiated  concretions  of  pyrite  or  marcasite  (0.5-1 
mm.  dia.)  are  occasionally  found. 

Greater  magnification  (400-600  dia.)  shows  scattered  angular 
quartz  grains,  a  few  scales  of  muscovite  and  rarely  rounded 
zircons.  None  of  the  above  exceed  10-15  microns  in  length. 
The  bulk  of  the  clay  is  made  up  of  plates  or  grains  less  than 
2  microns  diameter,  many  of  which  show  low  interference  colors 
in  polarized  light.  From  their  similarity  to  determinable  plates 
in  other  sections,  they  seem  to  be  kaolinite.  Sixty  per  cent, 
of  the  clay  is  estimated  to  be  visibly  crystalline,  the  balance 
being,  in  all  probability,  mainly  colloidal  in  nature. 

Section  II. — This  shows  the  contact  of  flint  clay  with  the 
underlying  semi-plastic  fire  clay. 

The  part  of  this  section  representing  the  "semi-plastic" 
clay  is  more  coarsely  and  thoroughly  crystalline  than  is  that 
of  the  flint  clay  (Plate  I,  Fig.  2).  Some  areas  of  the  "semi- 


15  Except  where  especially  noted,  these  descriptions  are  of  thin  sections  cut  from  the 
solid  clay,  and  not  of  mounted  powders. 


STUDIES  OF  FLINT  CLAYS  AND  THEIR  ASSOCIATES. 


305 


PLATE  I. 


Fig.  1.— Section  of  flint  clay,  Strasburg,  O., 
between  crossed  nicols.  Dark  spots  are  pyrite. 
Large  light  grains  quartz,  small  ones  partly 
kaolinite  X  60. 


Fig.  2.— Section  from  Strasburg,  O.  Shows 
line  of  contact,  the  darker-  portion  representing 
flint  clay  and  the  lighter  portion  the  semi-flint 
clay  with  high  development  of  the  hydro-micas 
X  160. 


306  STUDIES  OF  FLINT  CLAYS  AND  THEIR  ASSOCIATES. 

plastic"  are  seen  to  be  more  highly  crystalline  than  others,  indi- 
cating varying  stages  of  recrystallization.  The  platy  grains  are 
so  interlocked  and  intergrown  that  there  seems  to  be  no  doubt 
of  their  recrystallized  nature.  Most  of  these  plates  and  aggre- 
gates resemble  muscovite  closely,  except  that  they  do  not  show 
as  strong  double  refraction  as  that  mineral.  It  was  noted,  how- 
ever, that  this  property  varies  in  different  individuals,  and  that 
in  a  single  section  crystals  may  be  seen  which  would  form  an 
unbroken  series  whose  birefringence  varies  from  that  of  musco- 
vite to  nearly  or  quite  that  of  kaolinite.  To  distinguish  these 
plates  from  muscovite  and  kaolinite,  they  will  be  mentioned 
as  hydro-micas  and  discussed  more  thoroughly  in  a  later  para- 
graph. 

Scattered  through  the  felt  of  hydro-micas  are  occasionally 
columnar  or  fan-shaped  aggregates  of  plates,  showing  low  in- 
terference colors,  low  index18  and  the  direction  of  greatest  elas- 
ticity cutting  across  the  plates.  The  optical  character  could 
not  be  determined  in  any  of  the  plates  in  this  section,  but  a  similar 
plate  in  Section  14  gave  a  negative  biaxial  figure.  These  proper- 
ties agree  with  those  given  by  Iddings  and  Johannsen  for  kaolin- 
ite. 

Hickling17  has  found  the  index  of  kaolinite  in  some  of  the 
English  china  clays  to  be  nearer  1.56  than  1.54.  Furthermore, 
A.  B.  Dick18  describes  one  kaolinite  which  is  optically  positive 
rather  than  negative  and  whose  mean  index  is  about  1.563, 
while  another  he  finds  is  negative.  From  this  he  concludes 
that  there  are  two  varieties  of  kaolinite.  Among  others  who 
have  described  various  forms  of  kaolinite  crystals  are  Johnson 
and  Blake,19  who  first  used  the  term  kaolinite,  Merrill,20  Reusch,21 


16  The  index  of  similar  plates  in  the  powder  of  clay  No.  1  was  fixed  at  about  1 . 54  by 
the  use  of  "index  liquids."     The  strength  of  birefringence  was  determined  by  measuring 
the  thickness  of  grains  cut  normal  to  the  basal  cleavage,  by  focusing  on  first  the  top  and 
then  the  bottom  of  the  grain,  reading  the  difference  on  the  micrometer  head  and  correct- 
ing for  index  to  give  the  true  thickness.     Knowing  the  thickness  of  a  grain  and  its  inter- 
ference colors,  the  birefringence  may  be  determined  with  fair  accuracy. 

17  China  Clay,  Its  Nature  and  Origin. 

18  Supplementary  Notes  on  the  Mineral  Kaolinite,   Mineralogical  Magazine,  XV,    p. 
124-7. 

19  Am.  Jour.  Sci.,  ii,  XLII. 

20  Non-metallic  Minerals. 

21  Neues  Jahrb.  fur  Min.,  1887,  II,  p.  70. 


STUDIES  OF  FLJNT  CLAYS  AND  THKIR  ASSOCIATES.  307 

Rieke,22  Cook,23  Mellor24  and  Ries.25  All  of  these  investigators 
have  recognized  the  peculiar  sheaf  and  fan-shaped  bundles  of 
more  or  less  hexagonal  plates.  It  seems  that  there  are  then 
kaolinites  of  different  optical  properties,  which  may  well  represent 
simply  different  members  of  an  isomorphous  series,  containing, 
perhaps,  small  amounts  of  alkalies  or  other  elements  in  place 
of  hydrogen  in  the  theoretical  molecule. 

The  prisms  and  fans  of  this  section  frequently  show 
"ribs"  or  plates  of  higher  index  and  birefringence  intergrown 
with  those  of  kaolinite  (Fig.  2).  These  "ribs"  show  practically 
every  grade  of  variation  between  kaolinite  and  muscovite. 


KAOLINITE-  PRISMS    WITH 
INTER-GROWN    HYDRO- M  I  CA  PLATES. 

Fig.   2.— Sketch. 

Irregular  and  often  corroded  quartz  grains  (under  80  microns 
dia.)  make  up  about  15  per  cent,  of  the  crystalline  material  of 
the  section.  Zircon  grains  are  frequently  to  be  seen.  Rather 
angular  cloudy  patches  (under  100  microns  dia.)  are  sometimes 
found  and  may  represent  the  remains  of  decomposed  feldspar 
fragments.  Microscopic  crystals  (octahedral)  of  pyrite  are 
abundant  near  the  contact,  where  they  were  formed,  probably 
as  the  result  of  the  reduction  of  iron  sulphate  in  solution  by 
organic  matter  from  the  clay. 

Section  IV. — This  was  prepared  from  the  powder  of  the 
upper  fire  clay  (plastic) ;  sixty  to  seventy  per  cent,  of  it  seems  to 
be  in  the  form  of  small  plates  or  scales,  some  of  which  are  musco- 
vite, but  more  show  the  lower  interference  colors  of  the  hydro- 
micas.  No  grains  were  seen  which  could  be  determined  with 
certainty  as  kaolinite.  Small  quartz  and  zircon  grains  are  pres- 
ent in  slight  quantities.  The  texture  and  mineral  content  of 


22  Sprechsaal,  XL,  1907. 

23  Clay  Deposits  in  New  Jersey,  N.  J.  Geol.  Surv.,  1878,  pp.  280-281. 

24  Trans.  Eng.  Ceram.  Soc.,  Vol.  VIII,   1908-9. 
23  Md.  Geol.  Surv.,  IV.  Pt.  III. 


308 


STUDIES  OF  FLINT  CLAYS  AND  THEIR  ASSOCIATES. 


the  powder  is  similar  to  those  of  the  powdered  plastic  clay  from 
Mt.  Savage  (compare  Section  XIX). 

Section  VI. — Prepared  from  the  powder  of  the  "semi-flint" 
clay  found  between  the  upper  plastic  and  flint  clays. 

The  powder  resembles  that  of  the  plastic  clay  except  that 
some  of  the  grains  are  not  composed  of  a  single  flake  or  plate, 
but  are  made  up  of  a  large  number  of  small  plates  still  tightly 
bound  together.  (Compare  with  powders  of  Olive  Hill  semi- 
flint  and  Mt.  Savage  plastic  clays.)  Individual  grains  do  not 
exceed  30  microns  diameter.  When  lying  flat  in  the  section, 
their  birefringence  is  very  low  and  they  may  be  distinguished 
from  kaolinite  only  by  their  higher  index  of  refraction.  When 
on  edge,  the  higher  interference  colors  mark  most  of  them  as 
hydro-micas  and  muscovite.  Only  a  few  plates  of  kaolinite 
were  seen.  These  show  an  index  of  refraction  of  about  1.54, 
and  become  dark  in  polarized  light.  A  few  quartz  and  zircon 
grains  were  also  noted. 

RESULTS   OF   DEHYDRATION  TESTS. 

The  microscopic  examination  of  these  clays  led  to  the  belief 
that  there  should  be  decided  differences  in  the  water  content 
of  the  flint,  semi-flint  and  plastic  clays.  In  order  to  test  this 
view  a  15  to  2o-gram  sample  of  each  type  was  powdered,  dried 
at  112°  C.  and  then  ignited  to  approximately  constant  weight, 
with  the  following  results : 


Per  cent, 
drying  loss 

Per  cent, 
ignition  loss 

Per  cent, 
total  loss 

Flint  clay  
Semi-flint  clay              

0.$ 
o.  56 

12.3 

8.  os 

I3-I 
8.6l 

o-5 

7.87 

8-37 

Plastic  clay  

O.I28 

7-33 

7-43 

For  purposes  of  comparison  a  direct  determination  of  the 
water  in  the  plastic  clay  was  made  according  to  the  Penfield 
method,  giving:  H2O  =  6.9%,  other  volatile  matter  (CO2,  SO,, 
etc.)  =  i.o  per  cent.,  total  loss  =  7.9  per  cent.  This  clay  was 
taken  for  the  Penfield  determination  because  it  contained  con- 


20  Had  been  previously  dried  on  a  radiator. 


STUDIES  OF  FLINT  CLAYS  AND  THEIR  ASSOCIATES.  309 

siderable  visible  pyrite,  and  was  expected  for  that  reason  to 
show  a  higher  percentage  of  volatile  matter,  exclusive  of  water, 
than  the  other  clays.  It  seems  safe  to  conclude  that  with  most 
of  the  clays  tested,  more  than  90  per  cent,  of  the  ignition  loss 
may  be  safely  considered  to  be  water. 

The  results  were  significant,  in  that  they  show  the  impossi- 
bility of  much  kaolinite  in  either  the  semi-flint  or  plastic  clays. 
Commonly  the  low  water  content  of  a  clay  may  be  attributed 
to  the  presence  of  excessive  amounts  of  anhydrous  minerals, 
but  microscopic  evidence  is  against  such  an  explanation  in  this 
case.  Since  the  samples  tested  represent  clays  ranging  from  the 
middle  toward  the  upper  boundary  of  the  bed,  the  results  seem 
to  throw  some  light  on  the  geologic  history  of  the  clays,  which 
consideration  will  be  taken  up  later. 

CLARION  COUNTY,  PENNSYLVANIA. 

In  this  county,  as  in  the  region  around  Strasburg,  Ohio, 
valuable  fire  clays  exist  at  several  horizons  ranging  from  the 
Mercer  to  the  top  of  the  Alleghany  series.27  The  clays  investi- 
gated were  collected  at  mines  located  from  five  to  seven  miles 
northwest  of  Clarion.  Some  specimens  were  taken  from  stock 
piles  and  others  secured  in  the  mine  tunnels.  The  geologic  re- 
lations at  one  of  the  mines  is  shown  in  the  sketch  (Fig.  3). 


DISTANCE  TO  SURFACE  7O  -90  FT 
DISTANCE  TO  OVERLYING  COAL  30-4OFT. 

LENSES  OF  FLINT  CLAV 

IN    SANDSTONE. 


Fig.  3. — Section  at  Clarion,  Pa. 

The  clay  lying  between  massive  sandstone  beds  is  all  non- 
plastic,  and  occurs  in  somewhat  lenticular  masses,  which  at 
times  grade  into  sandstone  if  followed  along  the  bed.  The 


27  Bull.  315,  U.  S.  Geol.  Surv. ;   Bull.  454,  U.  S.  Geol.  Surv. 


310          STUDIES  OF  FLINT  CLAYS  AND  THEIR  ASSOCIATES. 

thickness  varies  from  o  to,  perhaps,  16  feet.  Variations  of 
color  and  texture  may  be  noted,  mainly  in  crossing  the  bed 
vertically,  but  also  in  following  it  horizontally.  The  better 
grades  of  clay,  usually  found  toward  the  middle  of  the  bed, 
are  of  a  cream  or  buff-gray  color,  and  show  a  clean,  smooth 
conchoidal  fracture.  All  gradations  from  this  type  into  a  clayey 
sandstone  of  dull  gray  color  and  rough  fracture  may  be  found. 

Although  no  coal  lies  directly  above  this  clay,  it  seems  prob- 
able that  the  horizon  is  that  of  the  Lower  Kittanning  coal,  which 
is  sometimes  missing  in  this  locality.28 

The  strata  of  the  region  show  only  slight  evidence  of  dynamic 
disturbance. 

MICROSCOPIC     PETROGRAPHY     OF     CLAYS      FROM     CLARION     CO.,     PA. 

Section  VII. — This  section  is  from  a  very  smooth  buff  variety 
of  the  Clarion  clay  which  shows  a  tendency  to  split  into  flat 
slabs.  The  type  occurred  as  a  layer  12  to  18  inches  thick,  but 
is  now  worked  out. 

Under  the  microscope,  the  clay  is  seen,  in  the  main,  to  be 
finely  crystalline,  although  the  section  is  somewhat  clouded 
by  a  limonite  scum.  In  places,  recrystallization  of  knots  or 
bunches  of  kaolinite  prisms  has  pushed  the  scum  out  into  rims, 
which  may  be  noted  without  magnification  as  brownish  spots 
or  circles  upon  the  surface  of  the  clay.  Other  than  these  re- 
crystallization  structures,  the  section  presents  a  rather  even 
and  homogeneous  appearance. 

Kaolinite,  in  prisms  under  50  microns  length,  fans,  and 
vermicular  forms,  composes  from  65  to  70  per  cent,  of  the  clay. 
"Ribs  "of  the  hydro-micas  are  seen  in  only  a  few  of  the  prisms 
and  fans. 

Rhombohedrons  of  siderite  (FeCO3)  often  60  to  70  microns 
in  length  are  evenly  scattered  through  the  section.  They  seem 
to  have  been  among  the  first  crystals  to  form  after  the  sediments 
were  laid  down.  Other  less  common  minerals  are  quartz  in 
badly  corroded  grains  up  to  90  microns  length,  zircon  grains, 
rutile  in  needles,  tourmaline  grains,  and  a  few  small  recrystallized 
plates  of  muscovite. 


Bull.  315,  U.  S.  Geol  Surv. 


STUDIES  OF  FLINT  CLAYS  AND  THEIR  ASSOCIATES.  311; 

Section  VIII. — This  is  slightly  sandy  flinty  clay,  and  the 
section  is  similar  to  VII  in  general,  but  shows  no  siderite  and 
contains  more  and  larger  detrital  grains  of  quartz,  zircon  and 
tourmaline.  Mica  plates  are  slightly  more  abundant,  many 
of  them  seeming  to  be  recrystallization  products. 

Section  IX. — This  represents  a  variation  which  is  so  sandy 
that  it  is  discarded  at  the  mines.  It  comes  from  near  the  top 
of  the  bed  (Plate  II,  Fig.  i).  Quartz  grains  with  very  irregular 
outlines  make  up  about  50  per  cent,  of  the  rock.  The  groundmass 
is  similar  to  that  of  Sections  VII  and  VIII,  but  shows  little 
or  no  muscovite.  The  development  of  hydro-mica  is  more  marked 
than  in  any  of  the  foregoing  clays  from  Clarion. 

Section  X. — A  darker  somewhat  sandy  phase  of  the  Clarion 
clay,  which  is  mottled  with  occasional  black,  waxy  patches 
or  lenses. 

In  general,  this  section  resembles  Nos.  VII,  VIII  and  IX, 
but  has  few  of  the  spherical  recrystallization  forms.  There 
is  a  larger  amount  of  the  brown  or  gray  "scum"  which  may 
here  be  in  the  nature  of  organic  matter.  A  section  was  cut 
across  ,one  of  the  black  waxy  lenses  which  under  the  microscope 
was  seen  to  be  clearer,  extremely  fine,  crystalline  material. 
The  minerals  could  not  be  identified.  It  seems  that  contrac- 
tion of  the  clay  had  opened  slight  fissures  which  were  filled  by 
this  fine-grained  material,  probably  from  other  parts  of  the 
same  bed. 

Section  XI. — Typical  flint  clay,  Clarion,  Pa.  This  clay 
differs  from  No.  VII  mainly  in  showing  no  tendency  to  break 
or  cleave  in  any  one  direction  more  than  in  any  other.  It  is 
buff  in  color  and  has  a  medium  conchoidal  fracture. 

Under  the  microscope,  its  structure  resembles  that  of  No. 
VII,  but  there  are  more  of  the  recrystallization  features,  two 
generations  being  visible.  (Compare  with  Sections  XX  and 
XXI.) 

Kaolinite  occurs  abundantly,  (i)  filling  the  clear  spaces 
in  the  recrystallization  spots,  (2)  in  prisms,  and  (3)  in  irregular 
felts  throughout  the  section. 

The  prisms  are  frequently  ribbed  with  hydro-micas.  One 
or  two  instances  were  noted  where  a  mineral  grain  determined 


312 


STUDIES  OF  FLINT  CLAYS  AND  THEIR  ASSOCIATES. 


PLATE  II. 


Fig.  1.— Section  of  sandy  flint  clay  from 
Clarion  county,  Pa.,  between  crossed  nicols. 
Shows  edges  of  two  corroded  quartz  grains.  The 
long  grain  seen  near  the  center  of  the  section  is 
a  kaolinite  prism.  A  few  mica  flakes  may  be 
seen  as  needle-like  grains  X  200. 


Fig.  2.— Soft  clay,  Woodland,  Pa.,  between 
crossed  nicols.  Shows  coarse  mat  of  plates  with 
scattered  muscovite  grains  or  flakes  X  60. 


STUDIES  OF  FLINT  CLAYS  AND  THEIR  ASSOCIATES.  313 

as  epidote,  representing  probably  the  recrystallization  of  traces 
of  calcium  with  the  silica  and  alumina,  lay  toward  the  center  of 
one  of  these  prisms  and  on  either  side  of  it,  the  ribs  being  de- 
veloped to  an  especially  strong  degree.  Some  kaolinite  fans 
were  noted  which  were  ribbed  with  plates  showing  the  birefring- 
ence of  muscovite.  In  all  cases  these  plates  are  clear  cut,  some- 
times extending  beyond  the  kaolinite,  and  showing  no  evidence 
of  decomposition  into  kaolinite.  The  evidence  seems  more 
to  favor  the  view  that  the  hydro-micas  have  developed  with  the 
kaolinite. 

DEHYDRATION    TESTS    OF    CLARION,    PA.,    CLAYS. 

Inasmuch  as  the  various  phases  of  the  Clarion  flint  clay 
do  not  show  much  miner alogic  difference,  it  was  not  expected 
that  there  would  be  so  marked  variation  in  their  water  content. 
Only  two  samples  were  dehydrated,  one  being  the  smooth  buff 
clay  described  under  Section  VII  and  the  other  the  sandy  phase 
(Section  IX).  The  results  of  these  tests  are  given  in  the  follow- 
ing table : 


:       •    U 

Per  cent, 
loss  112° 

Per  cent, 
loss  ignition 

Total 

7  Flint  clay  
9  Sandy  flint 

0-55 
o  03 

13-45 

12     17 

14.0 
1  2    2O 

The  variation  found  is  due  mainly  to  the  greater  per  cent, 
of  quartz  in  the  sandy  clay.  Carbon  dioxide  is  responsible 
for  some  of  the  percentage  loss  in  No.  VII  as  it  contains  numerous 
grains  of  iron  carbonate. 

WOODLAND,  CLEARFIELD   COUNTY,  PENNSYLVANIA. 

The  clay  underlying  the  Mercer  coal  is  economically  the 
most  important  in  this  locality29  although  other  horizons  produce 
considerable  amounts  of  refractory  material. 

Near  Woodland  this  bed  outcrops  near  the  base  of  the 
hills  and  not  many  feet  above  stream  level.  It  lies  in  a  nearly 
horizontal  position,  but  has  been  affected  somewhat  by  the 
folding  and  faulting  without  changing  the  nature  of  the  clay 
to  any  extent.  The  No.  i  mine  of  the  Harbison- Walker  Company 


29  Bull.  285,  U.  S.  Geol.  Surv.,  Clays  and  Shales  of  Central  Pennsylvania. 


314 


STUDIES  OF  FLINT  CLAYS  AND  THEIR  ASSOCIATES. 


is  crossed  by  one  fault  whose  throw  varies  from  8  to  40  feet. 
It  extends  in  a  northwest  direction  or  normal  to  the  axis  of 
general  folding  in  the  region,  and  is  said  to  continue  at  least 
two  miles  to  the  southeast  where  it  is  encountered  in  another 
mine. 

An  ideal  section  given  in  Fig.  4  may  serve  to  show  the  oc- 
currence. 


BLACK  SLICKENSIDED  CLAY 
OVERLAIN   BY  FEW  INCHES  OF 
COAL. 
0-14- FT. 


;'.:^y:XAy/j:V:/.^  SANDSTO^Ev' -.'•;.': : :  ••'•.]  ;.';'•  •'.•'•';'':.  •  •'•'• 


Pig.  4. — Ideal  section  at  Woodland,  Pa. 

(1)  The   "soft"  clay  is  a  gray  laminated  rock  with  rough 
fracture   and   cut  by  numerous   interwoven   slip   planes.     It   is 
fairly  smooth  but  contains  many  flakes  of  mica  which  are  quick 
to  catch  the  eye.     Though  not  truly  plastic  it  is  said  to  be  a  good 
bond  clay.     Its  boundary  with  the   flint  clay  is  irregular  and 
often  difficultly  de terminable. 

(2)  The   flint  clay  is  also  gray,   often  mottled  with  dark, 
irregular  blotches,  giving  it  the  appearance  of  a  breccia.     The 
fracture  is  roughly  conchoidal,  and  the  mass  is  broken  by  ir- 
regular seams  or  joints  whose  sides  are  usually  coated  with  limo- 
nite  and  sometimes  clear  tabular  crystals  of  barite.   The  occurrence 
of  barite  with  fire  clays  is  somewhat  unusual,  but  was  noted  also 
at  Olive   Hill,    Ky.     The   crystals   were   probably   deposited  by 
the  reaction  of  pyrite  on  solutions  of  barium  hydrate  or  carbonate. 
The  bed  is  variable  in  thickness,    and  to  some  extent  in  quality. 
One  instance  was  noted  where  the  clay  is  completely  cut  out 
by  the  overlying  sandstone  which  appears  to  have  settled  and 
squeezed  the  clay  out  as  its  surface  was  crossed  by  a  series  of 
rough  steps  (Fig.  4) . 


STUDIES  OF  FLINT  CLAYS  AND  THEIR  ASSOCIATES.          315 

Above  the  flint  clay  there  is  usually  a  thin  layer  of  smooth 
l)lack  clay,  with  a  few  inches  of  coal  at  the  top  and  over  this 
some  20  feet  of  material  called  "Black  Shelly."  This  is  a  dark 
shale  which  breaks  up  into  thin  planes  on  exposure  to  the  atmos- 
phere. The  sandstone  is  normally  found  above  this  shale. 

In  one  instance,  a  more  plastic  clay  is  found  above  the  flint 
clay  and  it  is  interesting  to  note  that  water  is  reported  more 
abundant  in  its  vicinity. 

MICROSCOPIC    PETROGRAPHY    OF  THE   WOODLAND   CLAYS. 

Section  XIV. — "Soft  clay,"  Woodland,  Pa.  This  specimen 
is  one  of  the  most  interesting  of  those  examined  microscopically. 

With  low  powers  (magnification  40  to  80  diameters)  it  is 
seen  to  be  highly  crystalline,  and  made  up  of  a  felt  of  mineral 
plates,  which  show  a  variation  of  from  5  to  200  microns  in  length 
or  diameter.  The  clay  is  not  microscopically  homogeneous, 
that  is,  there  are  areas  or  patches  which  seem  clearer  than  others 
(compare  Section  XV),  and  these  often  show  slightly  lower 
interference  colors,  although  the  felty  structure  is  similar  to 
other  parts  (Plate  II,  Fig.  2). 

There  are  three  prominent  types  of  mineral  grains:  (i) 
Large  plates  100-250  microns  (though  sometimes  smaller)  show- 
ing when  on  edge  parallel  extinction,  and  high  interference  colors. 
These  plates  are  fairly  evenly  distributed  through  the  section. 
Several  determinations  show  that  they  are  muscovite  and  may 
represent  detrital  material,  although  there  is  nothing  other 
than  their  size  to  indicate  this.  Frequently  these  plates  are 
bordered  by  an  irregular  growth  of  parallel  orientation  which 
shows  lower  index  of  refraction  and  lower  order  interference 
colors.  These  rims  do  not  seem  to  be  the  result  of  weathering 
because  of  their  irregular  shape  and  also  because  the  boundary 
between  them  and  the  muscovite  scales  is  clean  and  well  marked. 
The  muscovite  shows  good  cleavage  cracks  while  few  or  none 
are  to  be  detected  in  the  bordering  mineral.  They,  therefore, 
seem  to  be  additional  growth  of  hydro-mica  upon  the  muscovite 
and  indicate  simply  that  crystallization  of  the  border  took  place 
after  the  available  alkalies  for  the  formation  of  muscovite  had 
been  exhausted  (Fig.  5). 


316 


STUDIES  OF  FLINT  CLAYS  AND  THEIR  ASSOCIATES. 


B 


A-  MUSCOVITE    WITH    HYDRO-MICA  BORDER 

B- GROUP  OF  KAOLIN  ITE   PRISMS    SHOWING 
VARVING   AMOUNTS  OF  HVDRO-MICA  RIBS. 

Fig.  5.— Sketches. 

While  conspicuous,  these  plates  make  up  a  small  per  cent, 
of  the  clay. 

(2)  Plates  which  are  in  general  similar  to  (i)   but  smaller 
and  of  lower  index  and  birefringence  make  up  the  bulk  of  the 
section.     They  usually  lie  in  a  confused  felt-like  mass,  but  some- 
times show  parallel  orientation  over  considerable  areas.     (Com- 
pare  Section    XVII.)     Occasionally   they   form   fan-like    aggre- 
gates  closely   resembling   kaolinite   but   having   usually   higher 
index  and  birefringence.     These  are  believed  to  be  hydro-micas. 

(3)  Kaolinite  in  prisms,   fans  and  plates  is  present.     The 
commoner    shapes    are    represented    in    accompanying    sketches 
(Fig.  5,  B). 

"Ribbing"  is  always  present  to  a  considerable  extent  in 
these  kaolinite  prisms  (Plate  III,  Fig.  i).  Where  any  orien- 
tation could  be  detected  the  long  direction  of  the  prisms  lay 
parallel  to  the  direction  of  elongation  of  the  mica  plates,  which 
would  make  the  cleavage  of  one  perpendicular  to  that  of  the 
other.  The  tendency  sometimes  noted,  for  crystal  plates  and 
prisms  to  lie  with  their  long  directions  roughly  parallel,  prob- 
ably represents  movement  within  the  clay  under  pressure. 

Occasional  groups  or  bunches  of  kaolinite  prisms  irregularly 
set  together  may  represent  the  weathering  of  feldspar  fragments, 
although  it  seems  more  probable  that  they  are  remnants  of  the 


STUDIES  OF  FLINT  CLAYS  AND  THEIR  ASSOCIATES. 


317 


PLATE  III. 


Fig.  1.— Soft  clay.  Woodland,  Pa.,  by  direct 
ight.  Kaolinite  prisms  showing  ribs  of  higher 
index  of  refraction  X  300. 


Fig.  2. — Flint  clay.  Woodland,  Pa.,  between 
crossed  nicols.  Shows  only  the  larger  crystal 
grains  which  are  mainly  kaolinite,  often  "ribbed." 
A  few  bunches  of  the  hydro-micas  may  be  noted 
X  200. 


318  STUDIES  OF  FLINT  CLAYS  AND  THEIR  ASSOCIATES. 

knots  or  concretions  of  kaolinite  so  commonly  seen  in  the  more 
flinty  clays. 

Quartz  and  zircon  in  small  grains  are  present  but  incon- 
spicuous and  probably  rare. 

Rounded  tourmalines  were  noted  but  are  unimportant. 
Rutile  (?)  needles  of  small  size  seem  fairly  abundant,  some- 
times so  grouped  that  they  suggest  recrystallization  products 
from  the  weathering  of  some  mineral  such  as  biotite.  On  the 
whole,  there  is,  however,  no  definite  evidence  of  detrital  material 
other  than  quartz  and  zircon  in  this  clay. 

Section  XV. — Mottled,  or  "conglomerate"  flint  clay, 
Woodland,  Pa. 

This  section  is  composed  largely  of  visibly  crystalline  material, 
although  the  largest  individuals  rarely  exceed  20  microns  in . 
greatest  dimension.  The  average  size  of  grain  is  5  to  10  microns 
although  there  is  much  finer  material  (Plate  III,  Fig.  2).  The 
structure  throughout  is  fairly  uniform,  but  one  notices  that 
there  are  areas  clearer  than  others  which  with  slight  magni- 
fication resemble  sharp,  irregular  fragments  embedded  in  a  more 
turbid  ground  mass.  With  higher  magnification,  it  may  be 
seen  that  one  part  is  much  clearer  than  the  other,  although 
both  seem  equally  well  crystallized.  The  clear  areas  are  found 
by  comparison  with  the  rock  chip  from  which  the  section  was 
cut  to  represent  the  dark  patches  which  give  the  clay  its  brec- 
ciated  or  "conglomerate"  appearance.  Mineralogically  there 
are  some  differences;  The  clear  areas  are  composed  almost 
entirely  of  kaolinite  prisms  and  plates,  while  in  the  cloudy  parts 
the  incipient  hydro-micas  are  more  in  evidence.  Also  in  the 
cloudy  areas  knots  and  bunches  of  kaolinite  sometimes  occur, 
usually  with  a  concentration  of  the  limonite  scum  about  them, 
as  if  it  had  been  pushed  out  by  the  growing  crystals.  There 
are  places  too  where  this  scum  is  puckered  or  drawn  together 
about  centers  suggesting  the  contraction  of  a  colloidal  substance 
upon  drying.  The  breccia-like  structure  seems  due  to  a  con- 
traction in  the  clay  not  long  after  its  deposition  which  formed 
irregular  cavities  or  cracks.  These  were  then  filled  through 
infiltration  with  purer  material  derived  from  the  clay  itself. 
The  local  variations  in  minerals  noted  in  Section  XIV  may  be 


STUDIES  OF  FLINT  CLAYS  AND  THEIR  ASSOCIATES. 


319 


explained  as  the  metamorphic  equivalent  of  these  areas.  The 
minerals  in  order  of  their  importance  here  are  (i)  kaolinite, 
(2)  hydro-micas,  (3)  quartz,  (4)  zircon,  (5)  tourmaline,  (6)  rutile, 
(7)  mica. 

The  kaolinite  is  by  far  the  most  abundant,  the  lighter  areas 
being  made  up  of  the  small  crystals  of  it. 

The  hydro-micas  are  present  but  do  not  show  a  marked 
development.  They  occur  most  often  in  radiate  or  fan-shaped 
groups  of  not  more  than  4  or  5  plates,  each  of  which  are  seldom 
over  5  to  7  microns  in  diameter. 

Quartz  is  scarce,  as  are  also  the  other  remaining  minerals. 

Section     X VI.— "Semi-flint"     clay     from     Woodland,   Pa.30 

Microscopically  this  resembles  the  "semi-flints"  examined 
from  other  localities.  It  is  of  finer  texture  than  No.  XIV,  but 
coarser  than  XV.  Perhaps  90  per  cent,  of  the  section  is  visibly 
crystalline,  kaolinite  plates,  prisms  and  fans  with  hydro-micas 
making  up  its  bulk.  Sharp  sericitic  plates  are  in  evidence  be- 
cause of  their  higher  index  and  interference  colors.  The  section 
is  considerably  clouded  by  a  scum  of  limonite. 

Quartz  grains  are  scarce.  Rutile  (?)  needles  (under  2 
microns  length)  are  present  in  large  numbers  and  often  grouped, 
suggesting  the  weathered  residue  from  former  biotite  scales. 
Zircon  grains  were  also  noted  but  are  not  important. 

The  dehydration  of  two  samples  of  Woodland  clays  gave 
the  following  results : 

DEHYDRATION  TESTS  OF  WOODLAND,  PENNSYLVANIA,  CLAYS. 


Per  cent, 
loss  at  112° 

Per  cent, 
ignition  loss 

Total  per 
cent,  loss 

15  Mottled  flint  clay  
14  "Soft  clay"  

0-39 
0.39 

13-3 
12  .  II 

13-7 
I2-5 

0.58 

12  .  I 

12.7 

The  differences  here  are  not  so  great  as  those  shown  by  the 
Strasburg,  Ohio,  clays,  but  the  amount  of  kaolinite  seen  in 
Section  XIV  was  greater  than  that  of  the  other  semi-flint  clays. 

That  No.  XIV  owes  its  lower  water  content  to  the  develop- 


30  This  clay  comes  from  the  same  bed  as  do  Nos.  XIV  and  XV,  but  from  a  point  about 
to  2  miles  southeast. 


320 


STUDIES  OF  FLINT  CLAYS  AND  THEIR  ASSOCIATES. 


ment  of  hydro-micas  seems  probable  since  there  is  much  of 
this  mineral  to  be  seen  in  the  section,  and  greater  amounts  of 
quartz  or  other  anhydrous  mineral  than  in  No.  XV  are  not  noted. 
The  presence  of  hydro-micas  up  to  20  or  25  per  cent,  would 
produce  about  the  reduction  in  dehydration  loss  here  noted. 

MT.  SAVAGE,    MARYLAND. 

The  fire  clays  are  found  outcropping  toward  the  top  of 
Savage  Mountain  and  dipping  steeply  under  its  eastern  slope. 
They  are  overlain  by  the  Homewood  sandstone  of  the  Pottsville 
formation31  which  gives  them  a  stratigraphic  position  similar  to 
that  of  the  Woodland,  Pa.,  and  Olive  Hill,  Ky.,  flint  clays.  This 
clay  is  underlain  by  a  heavy  conglomerate.  The  Mt.  Savage 
coal  is  sometimes  present  above  the  clay32  but  in  the  mines  visited 
by  me  no  coal  was  seen. 


Fig.   6.— Section  at  Mt.  Savage,  Md. 


The  total  thickness  of  clay  shows  a  variation  from   2   to 
20  feet,  made  up  of  plastic  and  non-plastic  material.     The  flint 


31  Md.  Geol.  Surv.,  Report  on  Allegheny  County. 

32  Ries,   Clays  of  Maryland,  Md.  Geol.  Surv.,   IV,   Pt.  III. 


STUDIES  OF  FLINT  CLAYS  AND  THEIR  ASSOCIATES.  321 

clay  is  found  toward  the  middle  of  the  bed,  swelling  and  pinching 
with  great  irregularity.  Frequently  it  is  represented  only  by 
lenticular  masses  which  may  be  separated  from  each  other  along 
the  dip  by  a  distance  of  several  feet.  The  surfaces  of  these 
lenses  are  plainly  marked  with  slickensides,  giving  evidence 
of  considerable  movement  along  them.  The  clay  in  the  lenses 
is  usually  greenish  gray,  sometimes  mottled  and  broken  into 
irregular  chunks  by  cracks  or  joints  along  which  oxides  of  iron 
have  been  deposited.  Conchoidal  fracture  is  not  well  developed, 
and  in  general  appearance  this  clay  is  not  so  flint-like  as  the 
specimens  from  other  localities. 

The  plastic  clay  is  of  a  lighter  color  and  soft  enough  to  be 
broken  up  in  the  hands.  There  are  at  times  parts  which  are 
black,  due  to  the  presence  of  organic  material.  These  are  usually 
toward  the  top  of  the  bed  where  coal  is  said  to  be  found  in  some 
cases.  The  proportion  of  plastic  to  flint  clay  is  higher  than  in 
most  other  localities,  and  as  far  as  our  observation  goes,  the 
plasticity  of  the  soft  clay  is  greater  than  that  of  the  soft  clay 
from  other  localities  visited.  Because  of  the  geologic  structure 
of  the  region  the  clay  is  unusually  wet,  a  fact  which  may  have 
a  direct  bearing  upon  the  development  of  the  plastic  clay,  and 
which  will  be  discussed  later. 

Section  XVII.— Flint  clay,  Mt.  Savage,  Md.33 
When  examined  under  the  microscope  this  clay  is  similar 
to  No.  XVI  from  Woodland,  Pa.,  Few  well  defined  crystal 
grains  exceed  30  microns  in  greatest  diameter,  although  there 
are  frequently  areas  200  to  300  microns  across  through  which 
are  grown  many  more  or  less  distinct  plates  in  parallel  orienta- 
tion. Often  the  spaces  between  the  parts  of  one  such  group 
of  plates  are  filled  by  another  set  of  grains  in  different  orienta- 
tion from  the  former  but  parallel  to  each  other.  Such  structures 
may  well  represent  two  or  sometimes  more  intergrown  crystals 
although  it  is  not  thought  that  they  represent  twins,  since  there 
is  no  apparent  crystallographic  relation  between  the  position 
of  the  two  sets.  These  areas  are  so  irregular  that  they  prob- 


33  While  called  a  flint  clay  this  more  closely  resembles,   both  under  the  microscope 
and  in  the  hand  specimen,  the  "semi-flint"  clays  of  other  districts. 


322 


STUDIES  OF  FLINT  CLAYS  AND  THEIR  ASSOCIATES. 


PLATE  IV. 


Fig.  1. — Flint  clay,  Mt.  Savage,  Md.,  between 
crossed  nicols.  Shows  development  presumably 
by  hydro-micas  along  a  slip  plane  X  160. 


Fig.  2.— Semi-hard  clay,  Olive  Hill,  Ky.,  by  di 
rect  light.  Shows  cloudy  areas  which  may  repre- 
sent weathered  feldspars  X  80. 


STUDIES  OF  FLINT  CLAYS  AND  THEIR  ASSOCIATES.  323 

ably  are  not  caused  by  the  weathering  of  feldspar  grains  in 
situ,  but  represent  instead  intergrowths  of  rather  large  crystals. 

The  section  is  crossed  by  several  cracks,  which  evidently 
represent  slipping  planes,  as  indicated  by  the  drag  of  the  minerals 
along  them  (Plate  IV,  Fig.  i).  These  cracks  must  have  served 
as  passageways  for  solutions  for  the  reason  that  they  are  usually 
tinted  strongly  by  limonite. 

The  de terminable  minerals  are  (i)  kaolinite  in  plates,  prisms 
and  fans,  more  commonly  "ribbed"  by  the  hydro-micas;  (2) 
hydro-micas  in  individual  flakes  or  grouped  in  fan-shaped  aggre- 
gates. The  areas  described  above  are  composed  of  hydro- 
micas.  This  mineral  apparently  makes  up  from  50  to  60  per 
cent,  of  the  clay. 

Minute  rutile  needles  are  abundant,  small  zircon  grains 
common,  but  prisms  of  tourmaline  and  grains  of  quartz  are 
rare. 

Muscovite  scales  are  also  rather  rare  and  seldom  exceed 
15  microns  length.  An  unusual  reddish  mineral  was  noted  along 
the  slickensides  in  one  or  two  instances,  which  may  be  eucolite 
or  eudialyte.  The  evidences  are  not  satisfactory  for  certain 
identification. 

Section  XIX. — Prepared  from  powder  of  plastic  fire  clay, 
Mt.  Savage,  Md. 

The  powder  was  found  to  be  made  up  of  individual  crystal 
plates  and  grains  not  exceeding  20  to  25  microns  diameter,  while 
much  of  the  material  was  too  fine  to  determine.  It  is  interesting 
to  compare  this  powder  with  that  of  a  flint  clay,  and,  for  this 
purpose  a  section  of  the  "flint"  from  Strasburg,  Ohio,  was  pre- 
pared. It  was  made  up  of  irregular,  angular  grains  up  to  80 
microns  diameter.  As  both  clays  had  been  ground  the  same 
length  of  time  in  a  ball  mill,  this  difference  is  suggestive,  and 
probably  shows  why  the  "soft"  and  semi-flint  clays  develop 
greater  plasticity  than  do  the  flint  clays  as  they  are  more  readily 
reduced  to  very  fine  particles,  and  furthermore  the  platy  struc- 
tures of  these  particles  would  develop  greater  plasticity  than 
would  rounded  grains  of  equal  mass. 

Only  one  plate  of  kaolinite  could  be  identified  in  this  section,. 


324  STUDIES  OF  FLINT  CLAYS  AND  THEIR  ASSOCIATES. 

but  a  considerable  amount  of  the  material  was  shown  to  have 
an  index  of  about  1.54  by  the  use  of  index  liquids,  which  indicates 
the  presence  of  more  in  the  section.34  The  one  referred  to  was 
strongly  ribbed  by  hydro-micas.  The  hydro-micas  are  the 
most  common  minerals,  showing  indices  above  1.554  and  in  some 
cases  running  up  to  above  1.58,  which  would  indicate  that  the 
composition  was  approaching  that  of  muscovite,  scales  of  which 
are  also  common  in  the  section. 

Rutile  and  zircon  are  present,  the  former  being  more  fre- 
quently seen. 

Dehydration  tests  on  the  Mt.  Savage  clays  afford  results 
quite  similar  to  those  of  the  Strasburg  clays. 

DEHYDRATION  TESTS  OF  MT.  SAVAGE,  MARYLAND,  CLAYS. 


Per  cent,  loss 
at  112°C 

Per  cent, 
ignition  loss 

Total  per 
cent,  loss 

17  Flint  clay  

0.67 

11.72 

12  .4 

1  8  "Semi-flint"  
19  Plastic  clay  

0.66 
0.87 

7-37 
6-33 

8.03 
7-2 

It  may  be  noted  here  that  the  Mt.  Savage  flint  clay  was 
compared  microscopically  with  the  semi-flints  of  other  districts 
and  the  lower  water  content  would  seem  to  bear  out  the  similarity. 
The  "semi-flint"  and  the  "plastic"  clays  show  close  agreement 
with  similar  varieties  from  Strasburg.  The  samples  here  tested 
were  collected,  as  at  Strasburg,  ranging  from  the  center  of  the 
clay  bed  toward  the  hanging  wall. 

OLIVE  HILL,  KENTUCKY. 

The  geology  of  this  region  has  been  described  by  Greaves- 
Walker35  and  also  by  W.  C.  Phalen.36  The  former  has  given 
such  a  detailed  account  of  the  occurrence  of  the  clays  that 
it  seems  necessary  only  to  point  out  a  few  facts  about  the  region. 
The  following  section  from  Phalen's  report  is  characteristic: 


34  Quartz  is  also  of  about  this  index,  but  does  not  show  the  platy  structure  of  kaolinite. 

35  Trans.  A.  C.  S.,  Vol.  IX,    1907. 

36  Bull.  285,   U.  S.  Geol.  Surv.,  Clay  Resources  of  Northeastern  Kentucky. 


STUDIES  OF  FLINT  CLAYS  AND  THEIR  ASSOCIATES. 
SECTION  AT  OLIVE  HILL,  KENTUCKY. 


325 


Ft. 

In. 

Coal  

2-6 

No.  3  clay  

I—  Q 

Drab-flint  clay  

I—  Q 

Semi-hard  clay 

I  —  c 

Pink  ore  
Blue  shale  
Iron  ore 

1  8-20 

4-8 

Top    of    lower    carboniferous 
limestone  

There  is,  of  course,  considerable  divergence  from  this  section. 
In  the  "Burnt  House"  mine,  for  instance,  flint  and  semi-flint 
clays  only  are  found,  while  the  roof  is  a  massive  sandstone. 
At  the  same  horizon,  and  but  a  short  distance  from  some  of  the 
''Burnt  House"  workings,  is  found  the  peculiar  clay  described 
by  Greaves- Walker  as  "Aluminite"  rock.  It  seems  that  there 
must  be  a  transition  from  this  into  flint  clay  although  the  mine 
workings  are  not  reported  to  show  it.  It  is  also  possible  that 
it  may  represent  a  more  or  less  distinct  lens. 

The  Olive  Hill  clay  is  correlated  with  the  Mt.  Savage,  Md., 
and  Woodland,  Pa.,  clays,  and,  hence,  would  be  at  the  Mercer 
horizon  of  the  Potts ville. 

The  samples  examined  represent  (i)  a,  typical  flint  clay, 
b,  top  of  flint  clay  bed;  (2)  the  semi-flint;  (3)  the  aluminite 
varieties  described  by  Greaves- Walker.  All  are  found  in  or 
near  the  Burnt  House  mine. 

(ia)  (Specimen  No.  21)  is  of  buff  color  and  shows  a  good 
conchoidal  fracture.  Small  concretion-like  spots  may  be  seen. 
It  is  very  similar  in  appearance  to  some  of  the  Clarion,  Pa., 
flint  clays.  Fractures  in  this  clay  are  frequently  lined  with 
small  plates  of  barite. 

(ib)  (Specimen  No.  20).  This  clay  is  darker  than  No. 
21  and  shows  its  resemblance  to  the  typical  flint  clay. 

(2)  (Specimen  No.   22.)      A  smooth  buff-gray  clay  cut  up 
into    interlocking    lenses    by    slickensides.     The    central    parts 
of  these  lenses  are  like  the  flint  clay. 

(3)  (Specimen  No.  23.)     The  aluminite  is  a  buff  or  pinkish 


326 


STUDIES  OF  FLINT  CLAYS  AND  THEIR  ASSOCIATES. 


rock  of  great  toughness  having  a  sharp  but  rough  conchoid  al 
fracture,  the  surfaces  studded  with  oolites  like  those  seen  in 
bauxite,  but  of  small  size  and  seldom  exceeding  i  mm.  in  di- 
ameter. 

Section  XX.  —  Clay  from  the  top  of  the  flint  clay,  Olive  Hill, 
Kentucky. 

Structurally  this  clay  is  similar  to  No.  XI  from  Clarion, 
Pa.  It  is,  however,  richer  in  carbonaceous  matter,  which, 
along  the  cracks  that  served  as  pathways  for  solutions,  had 
precipitated  iron  in  the  form  of  pyrite.  Scattered  through 
the  clay  are  cloudy  areas  sometimes  of  circular  outline  and 
occasionally  bearing  remarkable  likeness  to  the  cross  sections 
of  feldspar  fragments  (Plate  IV,  Fig.  2).  Crystallization  of 
kaolinite  prisms  has  formed  clear  patches  in  some  of  these  areas, 
often  producing  radiate  structures  within,  and  more  frequently 
about  the  periphery  of  the  cloudy  area  (Fig.  7)  . 


/A''     •  ^T 


Fig.  7.  —  Types  of  structures  characteristic  of  Olive  Hill,  Ky.,  clays. 

a.  An  irregular  mass  of  kaolinite  prisms  developed  toward  the  center  of 
a  cloudy  oolite. 

b.  Kaolinite  grains  or  prisms  producing  radiate  structure  within  oolite. 

c.  Radiating  kaolinite  crystals  bordering  an  irregular  clouded  spot  (pos- 
sibly a  weathered  feldspar  fragment). 

Kaolinite  is  the  most  common  mineral  although  hydro- 
micas  are  to  be  seen,  especially  as  ribs  in  the  kaolinite. 

Quartz,  rutile,  zircon  and  tourmaline  are  present  in  lesser 
amounts. 

Section  XXI.  —  Flint  clay,  Olive  Hill,  Kentucky. 

This  section  closely  resembles  No.  XX  in  miner  alogical 
character,  but  is  free  from  pyrite  and  contains  much  less  carbon- 
aceous material,  and  the  structure  is  like  that  described  in  Sec- 
tion XI.  There  are  traces  of  a  sort  of  "mare's  tail"37  arrange- 
ment in  the  parts  least  crystallized.  These  structures  are  cut 


37  This  term  has  been  borrowed  from  meteorology  where  it  is  sometimes  used  to  desig- 
nate light  wispy  cloud  structures. 


STUDIES  OF  FLINT  CLAYS  AND  THEIR  ASSOCIATES.          327 

by  the  concretionary  and  cloudy  areas.  It  is  thought  possible 
that  these  structures  were  formed  by  movements  during  con- 
traction of  the  colloidal  jel,  of  which  the  clay  was  largely  com- 
posed, as  some  of  the  water  was  squeezed  from  it  by  the  weight 
of  accumulating  sediments  above.  Traces  of  such  structures 
are  to  be  found  in  all  of  the  very  fine-grained  flint  clays. 

Kaolinite  prisms  (up  to  70  microns  length)  show  a  great 
variety  of  bent  and  contorted  forms.  "Ribbing"  by  hydro- 
micas  is  a  prominent  feature  and  is  apparently  responsible  for 
much  of  the  warping  in  the  kaolinite  prisms.  Muscovite  is  prac- 
tically absent. 

Quartz  may  be  seen  in  widely  scattered  corroded  grains 
up  to  400  microns  diameter.  Other  accessory  minerals  are  as 
noted  in  other  sections. 

Section  XXII. — Prepared  from  powder  of  semi-flint  clay, 
Olive  Hill,  Kentucky. 

This  section  shows  largely  individual  plates,  but  also  many 
aggregate  grains,  indicating  that  the  development  of  hydro- 
micas  in  this  clay  is  not  as  complete  as  in  some  of  the  semi- 
flint  clays  from  other  localities,  for  the  development  of  hydro- 
micas  seems  to  accompany  any  movements  in  the  clay  which 
tend  to  loosen  up  the  individual  plates,  resulting  in  more  com- 
plete disintegration  into  individual  grains  upon  grinding. 

The  main  mineral  seen  is  hydro-mica.  No  kaolinite  was 
identified,  but  is  doubtless  present  to  some  extent. 

Muscovite  is  not  common,  and  the  same  is  true  of  quartz 
and  the  other  common  accessories. 

Section  XXIII. — Highly  aluminous  flint  clay,  Olive  Hill, 
Kentucky.38 

Here  one  sees  numerous  concretions  or  oolites  (Plate  V, 
Fig.  i)  scattered  through  a  fine-grained  ground  mass,  which 
is  similar  to  that  of  Sections  XX  and  XXI,  but  for  fewer  kaolinite 
prisms  and  occasional  patches  of  mineral  grains  which  show 
a  higher  index  than  kaolinite,  and  a  structure  differing  some- 
what from  that  of  mica.  Careful  determinations  of  elongation, 
extinction  and  birefringence  together  with  comparison  to  known 
hydrargyllite  (gibbsite,  (A1(OH)3)  lead  to  the  conclusion  that 


38  The  aluminite  of  Greaves-Walker. 


328 


STUDIES  OF  FLINT  CLAYS  AND  THEIR  ASSOCIATES. 


PLATE  V. 


Fig.  1. — So-called  "Aluminite,"  from  Olive 
Hill,  Ky.,  by  direct  light.  Shows  structure  of 
oolites  X  80. 


Fig.  2. — Section  of  "Aluminite"  from  Olive 
Hill,  Ky.,  between  crossed  nicols.  Shows  hy- 
drargyllite  (gibbsite)  recrystallized  in  central 
part  of  the  oolite  represented  in  Fig.  1  X  200. 


STUDIES  OF  FLINT  CLAYS  AND  THEIR  ASSOCIATES. 


329 


these  bunches  are  composed  of  grains  of  that  mineral  (Plate  V, 
Fig.  2).  It  is  more  common  in  the  oolites  than  in  the  ground 
mass.  Here  it  is  seen  in  nucleal  groups  and  concentric  bands 
separated  by  rings  of  cloudy,  extremely  fine-grained  material 
which  may  be  a  bauxitic  mixture. 

Kaolinite  prisms  are  rare  in  the  oolites  but  are  closely  as- 
sociated with  the  hydrargyllite  in  the  ground  mass.  The  hy- 
drargyllite  may  represent  the  recrystallization  of  hydrous  alumina 
which  was  present  originally  in  the  sediments. 

This  clay  seems  to  represent  an  intermediate  phase  between 
kaolin  and  bauxite. 

Dehydration  of  the  Olive  Hill  clays  gave  the  following 
results : 

DEHYDRATION   TESTS   OF    OLIVE   HILL,    KENTUCKY,  CLAYS. 


Per  cent,  loss 
at  112°C 

Per  cent, 
ignition  loss 

Total  per 
cent,  loss 

2  1   Flint  clay 

o  82 

I  IL      2 

14  o 

22  Semi-flint  

o  96 

117 

12    7 

23  "Aluminite".  

0.4 

13.0 

13-4 

The  "semi-flint"  shows  similarity  in  water  content  to  the 
Mt.  Savage  "flint"  and  the  Woodland  "soft"  clay. 

The  aluminite,  despite  the  lower  amounts  of  kaolinite  to 
be  seen,  shows  a  fairly  high  water  content  due  to  the  presence 
of  some  hydrargyllite  (A1(OH)3). 

OTHER  CLAYS  EXAMINED. 

For  purposes  of  comparison,  a  microscopic  examination 
was  made  of  the  following  clays : 

Missouri  flint  clay  (ground  section) . 
Washed  kaolin  from  Quebec  (prepared  from  powder). 
Washed  Delaware  kaolin  (prepared  from  powder) . 
Halloysite  from  Alabama  (prepared  from  powder) . 
Indianaite  from  Indiana  (prepared  from  powder) . 
Tennessee  ball  clay  (prepared  from  powder) . 
Plastic  New  Jersey  fire  clay  (prepared  from  powder) . 
St.  Louis  fire  clay,  hard  (prepared  from  powder) . 
St.  Louis  fire  clay,  soft  (prepared  from  powder) . 


330  STUDIES  OF  FLINT  CLAYS  AND  THEIR  ASSOCIATES. 

MISSOURI  FLINT  CLAY. 

This  is  a  cream-colored  clay,  having  a  fair  conchoidal  fracture 
and  a  more  earthy  luster  than  the  other  flint  clays  examined. 
The  mode  of  occurrence  has  been  briefly  pointed  out  in  the 
introduction  to  this  paper. 

Section  XXIV. — Flint  clay  from  Missouri. 

This  clay  is  structurally  like  Nos.  XX,  XXI  and  XI,  but  shows 
much  greater  development  of  the  "mare's  tail"  feature  (Plate 
VI).  The  texture  is  much  finer  than  that  of  any  other  clay  ex- 

PLATE  VI. 


Section  of  Missouri  flint  clay  between  crossed 
nicols.  Shows  the  mare's  tail  structure  and  the 
extremely  fine  grained  crystalline  texture 
X  300. 

amined,  there  being  comparatively  few  mineral  grains  of  size 
sufficient  for  identification. 

The  minerals  determined  are  (i)  kaolinite  prisms,  occasion- 
ally somewhat  "ribbed;"  (2)  minute  scales  of  muscovite;  (3) 
hydrargyllite  (gibbsite)  (?).  Of  this  last  mineral  we  cannot 
be  certain  as  the  grains  were  too  small  for  identification.  Their 
close  resemblance  to  those  seen  in  Section  XXIII  leads  us  to 
classify  them  as  such,  especially  as  the  Missouri  clays  often 
show  percentages  of  alumina  higher  than  that  of  kaolinite. 

Wheeler  has  noted  in  his  work  on  the  Missouri  clays39  that 


39  Mo.  Geol.  Surv.,  Vol.  XI,  1897. 


STUDIES  OF  FLINT  CLAYS  AND  THEIR  ASSOCIATES.          331 

the  water  content  of  the  flint  clays  often  exceeds  that  of  theo- 
retically pure  kaolinite,  and  suggests  that  it  may  be  caused  by 
the  presence  of  some  mineral  such  as  pholerite,  or  possibly  by 
the  admixture  of  kaolinite  and  bauxite.  The  latter  view  seems 
more  probable  from  the  examination  of  a  single  slide  and  also 
from  the  fact  that  of  the  hydrous  aluminous  minerals,  only  kaolin- 
ite, muscovite,  hydro-micas,  and  hydrargyllite  were  identified 
in  sections  of  several  of  the  eastern  flint  clays. 

WASHED  KAOLIN  FROM  QUEBEC. 

While  the  greater  part  of  this  powder  is  composed  of  very 
small  particles  there  are  numerous  crystalline  grains  which 
sometimes  reach  lengths  of  50  to  60  microns.  Many  of  these 
grains  are  platy,  with  low  index  of  refraction  and  the  optical 
properties  of  kaolinite  usually  seen.  Two  or  three  well  developed 
fans  of  the  same  mineral  were  noted,  but  it  is  probable  that  most 
of  these  fans  that  exist  in  the  crude  kaolin  are  broken  up  during 
the  process  of  washing.  A  few  good  tourmaline  prisms  were 
seen  and  occasional  flakes  of  mica. 

A  chemical  analysis  of  this  kaolin  showed  it  to  be  of  almost 
theoretical  purity.  Its  loss  on  dehydration  was  14  per  cent. 

WASHED  DELAWARE  KAOLIN. 

The  powdered  material  is  seen  to  be  composed  largely  of 
clear  transparent  plates,  the  largest  of  which  are  as  much  as 
100  microns  across.  Their  outlines  are  usually  irregular  but 
some  are  seen  which  are  hexagonal.  Some  of  the  smaller  grains 
are  more  fibrous  than  platy.  When  mounted  in  a  liquid  having 
a  refractive  index  of  1.54  their  edges  become  indiscernible,  show- 
ing that  the  index  of  the  plates  is  close  to  that  of  the  liquid. 
The  interference  colors  range  downward  from  grays  of  the  first 
order.  Extinction  is  nearly  parallel  and  in  some  cases  quite 
so.  A  few  of  the  plates  are  of  a  slightly  yellowish  color.  No 
fan  or  prism-like  aggregates  were  noted,  but  may  be  present 
in  the  kaolin  as  the  process  of  mounting  would  tend  to  break 
them  down.  The  mineral  is  doubtless  kaolinite. 

Some  mica-like  flakes  are  present,  as  are  also  numerous 
small  fibrous  bunches  which  show  the  properties  of  the  hydro- 
micas.  A  few  angular  quartz  grains  were  noted  as  well  as  some 
small  grains  taken  to  be  zircon  though  not  definitely  identified. 


332          STUDIES  OF  FLINT  CLAYS  AND  THEIR  ASSOCIATES. 

HALLOYSITE  FROM  ALABAMA. 

Numerous  platy  grains  are  seen  in  the  halloysite  powder, 
but  when  viewed  with  magnifications  of  600-800  diameters, 
these  grains  seem  to  be  composed  of  many  smaller  ones,  the 
optical  properties  of  which  could  not  be  determined  although 
they  seemed  to  be  weakly  doubly  refracting.  Some  small  plates 
of  muscovite  were  the  only  grains  which  could  be  identified 
with  certainty.  The  refractive  index  of  the  unidentified  material 
seemed  to  be  above  1.554,  suggesting  the  possible  presence  of 
hydrargyllite. 

Upon  dehydration  the  halloysite  lost  15.5  per  cent,  of  its 
original  weight. 

INDIANAITE. 

The  powder  is  mainly  crystalline  in  plates  and  grains  ranging 
from  80  microns  down.  The  plates  often  showaroughly  hexagonal 
outline  frequently  elongated  parallel  to  one  of  the  diagonals. 
They  have  but  slight  effect  upon  polarized  light.  There  are 
also  fans  or  bunches  of  plates  which  in  many  cases  show  the 
"ribbing"  so  characteristic  of  the  prisms  in  the  flint  clays.  The 
double  refraction  is  a  little  higher  than  that  usually  ascribed 
to  kaolinite,  but  the  agreement  of  other  properties  favors  this 
mineral. 

Quartz  and  zircon  grains  are  present  and  also  a  very  little 
greenish  mica.  Some  sharply  angular  fragments,  possibly  of 
opal,  have  been  noted.  About  60  to  70  per  cent,  of  the  Indianaite 
seems  to  be  made  up  of  crystalline  kaolinite  or  a  very  similar 
mineral. 

TENNESSEE  BALL   CLAY. 

This  powder  is  composed  of  apparently  crystalline  granules 
too  small  to  identify  (less  than  i  micron  diameter).  A  few 
grains  are  present  which  seem  to  be  zircons.  Much  of  the  material 
has  a  refractive  index  of  about  that  of  kaolinite  and  very  likely 
is  that  mineral. 

NEW  JERSEY  FIRE   CLAY. 

This  powder  is  of  finer  grain  than  that  obtained  from  any 
of  the  semi-flint  clays.  No  grains  were  seen  to  exceed  15  or 
20  microns.  The  coarser  grains  are  frequently  platy  and  oc- 
casionally show  roughly  hexagonal  outline.  Their  index  is 


STUDIES  OF  FLINT  CLAYS  AND  THEIR  ASSOCIATES. 


333 


below  1.554,  Dut  was  n°t  determined  more  definitely  than  that. 
They  are  thought  to  be  kaolinite.  Quartz  and  zircon  grains 
were  seen  as  well  as  a  few  muscovite  flakes. 

HIGHLY  ALUMINOUS  FIRE  CLAYS  FROM  NEAR  ST.  LOUIS. 

These  two  samples  were  sent  by  the  Laclede- Christy  Clay 
Products  Co.,  both  being  obtained  from  Missouri.  They  have 
the  following  chemical  composition,  as  determined  by  R.  T.  Hipp, 
Ceramist. 


178 

165 

SiO2               .            

45   45 

11   80 

Al  O 

-28    Q^> 

C  I       C-J 

Fe,(X  . 

I  .42 

I  .  17 

CaO 

O    I 

o  6  4 

MgO       

O  .  T.2 

O  .  14. 

Alkalies       . 

o  ST. 

0.68 

Water  (combined)  
TiO, 

12.35 

O    4Q 

12.99 

O    d-Q 

99.86 

99  •  74 

No.  178. — This  is  a  hard  gray  clay  (so-called  flint  clay) 
which,  however,  becomes  plastic  when  ground. 

The  powder  is  composed  of  minute  grains  of  crystalline 
appearance.  The  largest  do  not  exceed  15  microns  in  diameter. 
Only  a  few  grains  could  be  determined  among  which  were  quartz 
in  angular  grains,  mica  scales  and  a  few  flat  plates  resembling 
kaolinite. 

No.  165. — This  is  a  soft  clay,  the  sample  examined  being 
in  small  gray  pellets,  which  are  too  hard  to  be  crushed  between 
the  fingers. 

Much  of  the  powder  is  in  a  state  of  too  fine  division  for 
purposes  of  identification,  but  there  are  numerous  platy  grains 
which  sometimes  measure  25  microns  in  greatest  diameter.  These 
grains  are  irregular  or  more  rarely  somewhat  rhombic  in  out- 
line. The  refraction  index  is  considerably  above  1.54,  and 
the  birefringence  is  medium  to  high.  Extinction  seems  to 
take  place  parallel  to  the  diagonals  of  the  rhombs,  in  which 
the  longer  diagonal  is  the  direction  of  least  elasticity.  The 


334 


STUDIES  OF  FLINT  CLAYS  AND  THEIR  ASSOCIATES. 


prevalent  angles  in  the  rhombs  are  about  80°,  although  there 
is  considerable  variation  from  this  angle. 

It  seems  probable  that  these  grains  represent  basal  plates 
of  hydrargyllite  prisms. 

The  high  percentage  of  alumina  which  the  clay  shows  might 
in  itself  be  taken  as  evidence  of  the  presence  of  some  hydrate 
of  alumina,  but  if  this  excess  is  due  to  hydrargyllite  the  balance 
of  the  clay  must  be  rather  low  in  water  content. 

CHEMICAL  TESTS. 

Before  making  a  microscopic  examination  of  the  flint  clays 
it  was  thought  that  something  might  be  learned  concerning 
their  constitution  through  their  behavior  towards  acids.  Ac- 
cordingly several  of  the  clays  were  powdered  and  digested  with 
dilute  H2SO4  for  three  hours.  The  residue  was  then  filtered 
off  and  the  iron  and  alumina  precipitated  with  ammonia,  filtered 
off,  washed,  ignited  and  weighed.  Inasmuch  as  the  iron  present 
was  never  sufficient  to  more  than  tint  the  precipitate  it  was 
neglected.  The  precipitation  was  digested  with  HF,  and  again 
weighed,  the  figure  last  obtained  being  recorded  as  "soluble 
alumina." 

The  results  are  tabulated  below: 


Sample  No. 

J.-'.il.s- 

21 

22 

23 

4 

Halloy- 
site 

%Al2O3sol.  ini5%H2SO4.  .    |  9.57 

7-8| 

15.66 

27.44 

4-44 

3I-I 

i,  Flint  clay,  Strasburg,  O.;  21,  flint  clay,  Olive  Hill,  Ky. ;  22, 
semi-flint  clay,  Olive  Hill,  Ky. ;  23,  "aluminite,"  Olive  Hill, 
Ky. ;  4,  flint  clay,  Missouri ;  halloysite,  Alabama. 

Exclusive  of  the  ''aluminite"  and  halloysite,  the  solubili- 
ties shown  are  proportional  to  the  readiness  with  which  these 
clays  are  reduced  to  individual  grains. 

Washed  Delaware  kaolin  was  similarly  treated,  using  con- 
centrations of  10,  15,  20  and  25  per  cent.  H2SO4. 


Strength  of  acid 

10% 

15% 

20% 

25% 

%  soluble  A12O3 

A    4.6 

4C  C 

C      I 

6.540 

40  These  figures  represent  the  average  of  two  determinations. 


STUDIES  OF  FLINT  CLAYS'  AND  THEIR  ASSOCIATES.  335 

It  was  concluded  from  these  results  and  the  microscopic 
examination  that  the  solubilities  in  dilute  H2SO4  are  more  an 
evidence  of  the  fineness  of  the  powders  than  of  different  mineral 
content  except  in  cases  such  as  the  "aluminite"  which  has 
since  been  shown  to  contain  (A1(OH)3)  and  the  halloysite,  the 
mineral  composition  of  which  could  not  be  determined  because 
of  its  fine  texture. 

These  results  may  have  considerable  bearing  on  the  value 
of  sulphuric  acid  as  a  reagent  for  use  in  rational  analysis. 

It  may  be  seen  from  the  foregoing  data  that  the  assump- 
tion that  kaolinite  is  the  basic  mineral  of  all  highly  aluminous 
clays  is  in  a  few  cases  incorrect,  as  in  the  instance  of  those  "soft" 
or  semi-flint  clays  in  which  the  development  of  hydro-micas 
is  greatest.  No  evidence  has  been  found,  in  this  investigation, 
pointing  to  the  existence  of  any  other  hydrous  aluminum  silicates 
as  important  minerals  in  the  purer  clays  although  a  rather  wide 
range  of  materials'  has  been  included.  The  presence  of  alumina 
in  excess  of  percentages  allowable  for  kaolinite  may  frequently 
be  attributed  to  the  existence  of  hydrargyllite  in  a  clay,  and 
it  seems  possible  that  diaspore  (A12O3,  H2O)  may  also  occur  at 
times. 

Comparison  of  the  flint  clays  and  their  associates  with 
other  highly  aluminous  clays,  shows  that  the  main  difference 
is  in  texture.  In  the  flint  clays  the  kaolinite  crystals  are  knit 
together  so  tightly  that  when  the  clay  is  crushed  they  break 
out  in  aggregates  rather  than  as  individuals.  In  the  semi-flint 
clays,  the  crystals  though  larger  than  in  the  flint  clays  are  not 
so  tightly  interlocked  and  so  become  separated  on  grinding, 
producing  some  plasticity.  In  the  plastic  clays,  the  crystalline 
particles  are  smaller  and  at  the  same  time  more  or  less  separated, 
due  either  to  considerable  disintegration  of  a  once  crystalline 
clay,  or  to  a  lack  of  any  great  amount  of  original  crystallization. 
Kaolins  differ  from  the  flint  clays  in  having  crystallized  under 
conditions  of  less  pressure,  allowing  a  more  open  structure, 
which  may  account  for  the  fact  that  they  are  frequently  less 
pure  than  the  flint  clays  because  of  infiltration  of  foreign  matter. 
Any  attempt,  however,  to  do  more  than  point  out  general  causes 
for  differences  in  the  various  clays,  must  be  based  upon  a  most 


336          STUDIES  OF  FLINT  CLAYS  AND  THEIR  ASSOCIATES. 

intimate  acquaintance  with  all  of  the  different  deposits,  which 
it  has  not  been  the  writer's  good  fortune  to  acquire. 

THE  NATURAL  HISTORY  OF  THE  FLINT  CLAYS. 

The  origin  and  the  nature  of  the  flint  clays  have  always 
excited  interest,  and  yet  but  few  attempts  have  been  made 
to  explain  why  or  how  they  come  to  be  so  different  from  the  plastic 
clays. 

E.  Orton,  Sr.,41  Wheeler42  and  Hopkins43  have  advanced  the 
view  that  the  flint  clays  owe  their  hard  and  non-plastic  character 
to  part  or  complete  crystallization  of  kaolinite  in  them.  The 
soft,  semi-plastic  and  plastic  clays  associated  with  them  are 
considered  to  be  much  less  crystalline  and  finer  grained.  The 
investigations  described  in  this  paper  have  led  to  a  theory  of 
formation  which  agrees  only  in  part  with  these  earlier  views. 

The  land  surface  from  which  the  clay-forming  sediments 
of  carboniferous  times  were  derived  was  presumably  rather 
low  and  traversed  by  sluggish  streams.  The  rank  types  of  vege- 
tation which  flourished  are  taken  to  be  evidence  of  a  warm  climate, 
perhaps  even  subtropical  in  nature.  The  rocks  which  were 
being  disintegrated  and  decomposed  were  mainly  sedimentaries 
of  Devonian  and  Silurian  age.  The  weathering  processes  which 
were  then  active  were  more  or  less  such  as  are  active  at  the 
present  time  in  hot  moist  climates,  producing  lateritic  soils44 
which  were  richer  in  alumina  and  iron  than  soils  of  temperate 
climates.45  As  these  soils  were  carried  away  by  streams  to  be 
deposited  in  bogs,  swamps  or  lagoons,  part  of  their  iron  content 
was  dissolved  by  carbon  dioxide,  resulting  from  vegetable  decay 
in  the  water.  As  the  sediments  were  laid  down  they  became 
the  soil  in  which  the  coal-forming  plants  flourished  and  here 


11  Geol.  Surv.  Ohio,  VII,  Part  1. 
42Geol.  Surv.  Mo.,  XI. 

43  "Clays  of  Western  Pennsylvania,"  Ann.  Rept.  Penn.  State  College,  1897. 

44  Lateritic  weathering  has  been  shown  by  many  investigators  to  result  in  a  reduc- 
tion of  silica,  alkalies  and  alkaline  earths  and  a  concentration  of  alumina.     See  M.  Bauer 
Neues  Jahrb.  Min.  Geol.,   1907,  p.  33-90;  Chaubard  and  Lemome,  Compt.  rend.,  146,  p, 
239-42.     Others  have  found  kaolinite  to  be  formed  also,  E.  C.  Mohr,  Bull.  Dept.  1'Agric. 
Ides.  Neerld.  No.  28;  R.  Lenz,  Neues  Jahrb.  Min.  Geol.,  1909,  II,  347. 

4t>  The  deeply  corroded  quartz  grains  so  frequently  noted  in  the  flint  clays  may  be 
taken  as  a  further  evidence  of  lateritic  weathering. 


STUDIES  OF  FLINT  CLAYS  AND  THEIR  ASSOCIATES.          337 

they  were  further  purified  by  the  leaching  action  of  plant  roots 
and  the  carbonated  waters  of  the  bogs. 

It  may  be  pointed  out  then  that  the  sediments  of  the  clay 
beds  must  have  been  rather  free  from  iron,  calcium,  magnesium, 
potassium,  and  sodium  when  laid  down,  due  to  their  having  been 
reworked  and  to  the  fact  that  the  waters  of  the  carboniferous 
streams  were  probably  unusually  high  in  their  content  of  carbon 
dioxide.  If  we  consider,  further,  that  these  clays  usually  served 
as  soil  in  which  coal  vegetation  was  rooted,  further  purification 
is  to  be  expected  for  it  has  been  shown  that  plant  roots  are  very 
active  in  removing  lime,  potash,  and  also  magnesia,  soda  and 
iron  to  lesser  content. 

It  then  seems  probable  that  when  first  laid  down  these 
clays  were  very  fine  grained,  hydrous  aluminous  silicates  with 
varying  amounts  of  hydrous  alumina  as  a  result  of  the  lateritic 
weathering.  The  amounts  of  impurities  were  probably  as  low 
as  at  present,  when  consolidation  began,  for  it  does  not  seem 
possible  that  there  could  be  much  circulation  of  leaching  solutions 
through  the  settling  clay  mass. 

Every  fine-grained  sedimentary  deposit  must  include  a 
large  per  cent,  of  water  when  first  laid  down,  and  this  would 
be  especially  true  of  a  clay  bed.  As  soon,  however,  as  any 
pressure  was  brought  to  bear  the  pore  water  would  begin  to 
filter  out,  carrying  with  it  some  soluble  material.  Eventually 
only  enough  water  would  remain  to  fill  the  spaces  between  the 
component  particles  of  the  clay  provided  no  changes  took  place 
in  these  particles.  Changes,  however,  do  take  place  since  the 
equilibrium  of  the  surface  weathering  zone  is  not  the  same  as 
that  of  the  buried  deposit.  Recrystallization46  then  sets  in, 
resulting  in  a  compacting  of  the  mass;  and  in  some  cases  the 
excess  water  collects  in  fissures  and  cracks  formed  by  the  con- 
traction attendant  upon  recrystallization.  These  fissures,  under 
increasing  load  of  overlying  sediments,  become  filled  by  the 
squeezing  in  of  fine  particles  and  colloidal  material  along  with 
the  water.  The  brecciated  appearance  of  the  Woodland,  Pa., 


40  According  to  Doelter  (Min.  Pet.  Mitth.,  XXVIII,  p.  557-9)  amorphous  substances  (such 
as  colloidal  A1(OH)3,  etc.)  may  be  made  to  crystallize  by  digesting  at  60 °-70°  with  water. 
It  seems  quite  probable  that  the  heat  produced  by  decomposition  in  the  overlying  coal  or 
peat  was  sufficient  to  bring  about  recrystallization  in  the  underlying  clay. 


338          STUDIES  OF  FLINT  CLAYS  AND  THEIR  ASSOCIATES. 

clay,  and  the  dark  waxy  lenses  frequently  seen  in  other  flint 
clays  seem,  in  my  opinion,  to  represent  such  changes.  The 
filling  material,  as  seen,  usually  contains  fewer  impurities  than 
the  surrounding  clay.  This  may  be  explained  on  the  ground 
that  the  small  amount  of  alkali  present,  when  the  fine  sediments 
were  laid  down,  was  taken  up  to  form  sericite  and  hydro-micas 
during  the  early  stages  of  recrystallization.  The  hydrates  of 
iron  in  the  presence  of  a  considerable  amount  of  carbon  dioxide 
formed  siderite  (as  seen  in  Section  VII) ;  and  it  is  of  interest 
to  note  that  sericitic  scales  are  practically  wanting  and  hydro- 
micas  less  common  in  this  than  in  other  flint  clays  due,  it  may 
be  presumed,  to  the  removal  of  the  alkalies  by  the  excess  of 
carbon  dioxide.  When  iron  carbonate  was  not  formed,  most 
of  the  iron  became  enmeshed  in  the  matte  of  growing  crystals, 
remaining,  as  seen  in  many  sections,  as  a  sort  of  scum  in  the 
less  altered  parts  of  the  clay.  The  fixation  of  these  impurities 
left  the  material  which  filled  the  skrinkage  cracks  in  a  purer 
condition,  so  that  on  recrystallization  a  nearly  pure  kaolinite 
mass  resulted.  The  noticeably  white-burning  character  of  the 
"dark"  patches  is  explained  on  these  grounds. 

The  crystallization  of  kaolinite  and  hydro-micas  in  the 
flint  clays  has  gone  on  at  about  the  same  time  as  is  indicated 
by  the  fact  that  many  of  the  kaolinite  prisms  contain  plates 
of  the  higher  index  mineral.  From  all  the  evidence  I  could 
gather  through  examination  of  numerous  thin  sections,  there 
seems  to  be  an  isomorphous  or  nearly  isomorphous  series  of 
minerals,  ranging  from  kaolinite  through  to  muscovite.  Some- 
times in  a  single  prism  there  may  be  seen  sections  or  plates  whose 
optical  properties  vary  between  those  of  the  aforesaid  minerals 
as  limits.  Usually  the  plates  of  higher  index  are  distributed 
through  the  central  portion  of  a  prism,  indicating  that  the  kaolin- 
ite continued  to  form  after  the  available  alkalies  had  been  used 
in  the  growth  of  muscovite  or  the  hydro-micas.  There  is  not, 
however,  any  appearance  of  growth  of  the  kaolinite  at  the  ex- 
pense of  the  micas.  Hickling47  has  described  kaolinite  prisms 
from  some  of  the  English  china  clay  deposits  which  show  a  varia- 
tion from  kaolinite  to  mica.  He  considers  that  there  the  musco- 


47  "China  Clay,  Its  Nature  and  Origin."  Trans.  Inst.  Min.  Engrs.,  Eng.  XXXVI,  p.  10, 
1908-9. 


STUDIES  OF  FLINT  CLAYS  AND  THEIR  ASSOCIATES.  339 

vite  represents  a  transition  stage  in  the  weathering  of  feldspar 
to  kaolin,  which  is  doubtless  correct. 

The  apparent  transition  from  sericite  to  kaolinite  has  been 
noted  in  other  instances.48 

Although  the  empirical  formulae  commonly  used  for  the 
micas  and  kaolinite  do  not  give  great  resemblance,  it  is  possible  to 
so  construct  them  that  they  will  show  much  similarity  as  follows : 

Kaolinite  3(H4Al2Si2O9) 
OH 

Ji  .-;•."•• 

/\ 

o    o 


X) 

/\  I  I  /OA1/ 

OH— Al<       >Si  Si<          XOH 

^O/  |  |    X)H 

O  O  /OH 

,0,     |  |  /OM/ 

OH— Al<       >Si  Si<          XOH 

X0/    |  |    XOH 

O— Si— O 


O      OA1 

/        I    \ 
H       OH    OH 

Second  Kaolinite  Arrangement 
OA1(OH)2 


X 


0  ^ 

1  /V 

(OH)2  =  A1O— Si /  NSi— O— Al=  (OH)2 

I  /\ 

O  O      O 


(OH)2A1— O— Si  \  /Si— O— Al  =  (OH), 

I       °\ 
O \ 

O— Al  =  (OH)a 


V.  Selle,  Chem.  Cent.,  1908,  11,  1903. 


340          STUDIES  OF  FLINT  CLAYS  AND  THEIR  ASSOCIATES. 

Muscovite  2(H2KAl3Si3O12) 
K 
O 

Al 

/\ 
O      O 


HO— Al<        >Si  Si<       >A1— OK 

I  IX0/ 

0  O 

1  I  /°\ 

HO— AK'      >Si  Si<      >A1— OH 

IX0/ 
O 


/\ 
O      O 


Al 

OH 

Although  such  structural  symbols  are  largely  speculative, 
they  are  given  because  they  are  suggestive.  The  change  from 
muscovite  to  kaolinite  is  accomplished  through  the  replacement 
of  two  (KO)  groups  by  two  (OH)  and  the  addition  of  3H2O. 
In  a  molecule  of  the  size  postulated  such  an  addition  would  not 
of  necessity  change  either  form  or  molecular  volume  to  a  great 
extent. 

Mellor  and  Holdcroft4'  have  classed  muscovite  chemically 
with  kaolinite  saying  that  in  muscovite  there  appears  to  be  an 
addition  of  three  kaolinite  groups  where  two  (OH)  are  replaced 
by  equivalent  (KO)  groups.  They  propose  a  formula  for  kaolinite, 
however,  which  joins  all  of  the  hydroxyl  groups  with  the  alumina 
thus: 

(HO),  -  Al  —  O.SiO 
[H0)a  =  Al  —  O.SiO 


49  "The  Chemical  Constitution  of  the  Kaolinite  Molecule,"  Trans.  Eng.  Cer.  Soc.,  Vol.  X. 


STUDIES  OF  FLINT  CLAYS  AND  THEIR  ASSOCIATES.          341 

This  arrangement  is  based  largely  on  the  fact  that  dehydration 
curves  for  kaolinite  show  no  discontinuities  of  constitution. 

Some  other  investigators  have  favored  distributing  three 
hydroxyl  groups  between  both  Si  and  Al.  Among  these  are 
F.  W.  Clarke,50  F.  Hundeshagen,51  W.  Vernadsky,52  G.  Simmonds,53 
and  W.  Pukall.54  P.  Groth55  and  F.  Ulffers56  favor  symbols  similar 
to  those  of  Holdcroft  and  Mellor. 

The  second  kaolinite  formula  would  conform  somewhat  to 
the  ideas  of  the  latter,  but  the  objection  is  apparent  that  the 
substance  should  be  hexagonal  (trigonal)  with  such  a  symbol. 
In  either  case  the  possibilities  of  isomorphism  between  kaolinite 
and  muscovite  seem  fair.  On  the  other  hand,  it  may  be  that 
isomorphism  extends  only  to  the  complete  replacement  of  K 
by  H;  and  that  with  the  addition  of  water  a  molecule  is  built 
up  which,  though  not  capable  of  forming  isomorphous  mix- 
tures with  the  micas,  is  sufficiently  similar  to  be  oriented  through 
contact  with  them.  I  am  inclined  to  believe  in  complete  iso- 
morphism since  the  analyses  of  muscovites  show  considerable 
variation  of  hydrogen  and  alkali  content,  water  frequently 
being  present  up  to  6  or  61/2  per  cent,  often  as  an  addition  in 
part  but  occasionally  replacing  the  alkali.  To  my  knowledge, 
no  mica  has  been  reported  in  which  the  alkalies  have  been  totally 
replaced,  yet  such  an  occurrence  does  not  seem  impossible  or 
improbable  except  for  the  fact  that  alkalies  are  usually  present 
to  a  slight  extent  where  micas  are  forming.  Such  a  mica  would 
have  the  theoretical  composition:  H3Al3Si3O12,  or  in  per  cents., 
H2O  =  7.5,  A12O3  =  42.4,  SiO2  =  50. i.57  There  would  then  be 
two  classes  of  micas  showing  water  content  in  excess  of  the  theo- 


50  "Constitution  of  the  Silicates,"  1896. 

51  Zeit.  angew.  Chem.,  Vol.  XXI,   1908. 

52  Zeit.  Kryst.,  Vol.  XXXIV,  p.  37,  1901. 

53  Jour.  Chem.  Soc.,  Vol.  LXXXIII,  p.  1449,  1903. 

54  Chem.  Zeit.,  Vol.  XXXIV,  610-13. 

55  "Tabellarische  Uebersicht  der  Mineralien,"  p.  137,  1898. 
50  Jour,  prakt.  Chem.  [2],  LXXVI,  p.  143,  1907. 

57  Brackett  and  Williams  (Am.  Jour.  Sci.,  XLVII,  ii,  21,  1891)  have  described  a  min- 
eral which  they  call  rectorite,  having  a  theoretical  composition  of  H2Al2Si2Og  (A^Os  42.52 
per  cent.,  SiO2  49.99  per  cent.,  H2O  7.4  per  cent.),  which  is  biaxial,  strongly  birefringent, 
with  a  refractive  index  below  balsam  (1 .54).  The  properties  of  the  hydro-micas  are  very 
similar  to  this  mineral,  except  that  the  refractive  index  in  them  is  considerably  above 
that  of  balsam. 


342  STUDIES  OF  FLINT  CLAYS  AND  THEIR  ASSOCIATES. 

retical  amounts:  (i)  the  hydrated  muscovites  in  which  the  excess 
water  is  in  the  nature  of  an  addition,  and  (2)  the  hydro  musco- 
vites in  which  the  water  replaces  alkalies.  It  seems  probable 
that  both  types  are  represented  in  the  flint  clays,  when  the 
first  recrystallization  had  taken  place  in  the  presence  of  an  ex- 
cess of  water  and  a  want  of  sufficient  alkalies  for  the  formation 
of  muscovite  only. 

It  is  therefore  suggested  that  the  consolidation  of  the  largely 
colloidal  sediments  took  place  partly  as  the  settling  of  a  colloidal 
gel,  but  mainly  as  a  recrystallization  of  their  hydrous  aluminum 
silicates,  with  the  formation  of  minute  interlocking  crystals 
of  kaolinite  and  hydrated  micas,  resulting  in  the  formation  of 
the  flint  clays. 

During  and  after  the  "setting"  of  the  flint  clays  there  were 
forces  at  work  tending  to  break  them  down.  These  forces  were 
(i)  pressure,  both  from  the  weight  of  overlying  sediments  and 
from  compressive  movements  of  the  earth's  crust,  (2)  heat  de- 
rived partly  from  chemical  changes  that  were  taking  place  in 
the  coal  measures  and  in  part  from  the  friction  of  rock  structures 
as  they  became  readjusted  to  accommodate  the  changing  pressures. 
The  effect  of  heat  and  pressure  on  the  flint  clays  was,  first,  to 
break  up  the  outer  parts  of  the  bed  with  numerous  fractures 
along  which  there  was  some  slipping,  and,  second,  to  cause  a 
dehydration  and  metamorphism  of  the  parts  so  broken  up. 
The  extent  to  which  this  action  was  carried  depended  upon 
the  local  conditioas.  The  Clarion  clay  bed  was  apparently 
affected  but  little,  probably  because  of  the  massive  sandstone 
strata  above  and  below  which  protected  it  from  unequal  local 
pressures.  The  clays  at  the  other  localities  show  varying  amounts 
of  metamorphism.  At  Mt.  Savage,  where  pressures  were  prob- 
ably most  active,  the  flint  clays  often  occur  only  as  lenses,  the 
original  bed  having  been  broken  across  in  many  places  by  the 
local  faulting.  The  usual  irregular  boundary  between  the  "flint 
clay"  proper  and  the  more  or  less  plastic  clay  associated  with 
it  is  also  readily  explained  by  this  view.  The  metamorphism 
which  accompanied  the  fracturing  and  faulting  resulted  in  the 
development  of  hydro-micas  mainly,  especially  in  the  parts 
where  movement  and  friction  were  greatest.  That  the  kaolinite 


STUDIES  OF  FLINT  CLAYS  AND  THEIR  ASSOCIATES.          343 

has  lost  part  of  its  constitutional  water,  especially  the  smaller 
crystals,  seems  certain  from  microscopic  examination  of  the 
"soft"  and  semi-plastic  clays;  and  the  results  of  dehydration 
tests  show  rather  conclusively  that  the  water  content  of  some 
of  the  clays  is  close  to  the  theoretical  amount  required  for  a  pure 
hydro-mica. 

The  clays  of  this  metamorphic  type  are  known  at  various 
localities  as  "soft,"  "semi-hard,"  or  "semi-flint"  and  "semi- 
plastic"  clays.  They  show  greater  plasticity  than  the  true 
flint  clays  because  the  crystal  prisms  have  been  broken  down 
largely  into  plates  and  the  clay  as  a  whole  made  more  friable 
by  countless  small  faults,  so  that  with  equal  amounts  of  grinding 
the  metamorphosed  clay  affords  much  more  fine-grained  platy 
material  which  will  develop  some,  although  not  great  plasticity.58 

The  development  of  truly  plastic  clays  from  the  semi-flint 
varieties  has  sometimes  been  accomplished  through  weathering. 
After  much  of  the  overburden  was  removed  and  the  pressure 
on  the  clay  beds  reduced,  the  fault  planes  began  to  open,  allow- 
ing water  to  enter  and  begin  the  process  of  breaking  down  some 
of  the  mineral  grains,  at  the  same  time  usually  to  deposit  some 
iron  and  traces  of  lime,  magnesia  and  alkalies  in  the  numerous 
capillary  fissures.  The  continuation  of  this  process  for  a  suffi- 
ciently long  time  has  produced  rather  plastic  clays,  which  are 
at  the  same  time  less  pure  than  the  unweathered  clays.  Such 
clays  are  found  at  Mt.  Savage  and  also  at  Strasburg,  and  at  each 
locality  the  overburden  is  thin.  At  Mt.  Savage,  the  drainage 
is  such  as  to  facilitate  weathering  processes. 

It  may  be  of  interest  to  consider  somewhat  the  interpreta- 
tion which  may  be  made  of  results  obtained  by  others  in  experi- 
menting with  flint  clays. 

Wheeler59  gives  the  fire  shrinkage  of  the  Missouri  flint  clays 
as  ranging  from  9  per  cent,  to  14  per  cent. 


58  Cook  ("Clay  Deposits  in  New  Jersey,"  p.  281),  speaking  of  some  kaolin  clays,  says: 
"Trituration,  as  rubbing  between  the  fingers,  breaks  up  the  bundles  and  the  mass  is  ren- 
dered thereby  more  plastic  than  in  its  original  state."  With  the  semi-flint  clays  the  trit- 
uration  was  of  a  more  rugged  nature  but  equally  effective. 

*9  Loc.  cit. 


344          STUDIES  OF  FLINT  CLAYS  AND  THEIR  ASSOCIATES. 

Purdy60  records  fire  shrinkages  for  Olive  Hill  flint  clay  as 
9  per  cent,  at  cones  9-11  .and  13. 

Knote61  gives  fire  shrinkage  curves  for  Olive  Hill  flint  and 
plastic  clays,  Clearfield  County  (Pennsylvania)  flint  and  plastic 
clays,  Mt.  Savage  flint  clay  and  others. 

At  cones  11—13  they  run  as  follows: 

Clearfield  County  flint  clay 54% 

Mt.  Savage  flint    clay 5-6-5% 

Clearfield  County  plastic  clay 6.  2% 

Olive  Hill  plastic  clay 9 .4-9% 

Olive  Hill  flint  clay 9% 

If  the  Clearfield  County  flint  clay  tested  was  of  the  type 
described  in  this  paper  as  "soft"  clay,  the  increasing  shrinkage 
noted  by  Knote  is  found  in  clays  showing  increasingly  fine  and 
interlocking  grains.  The  Missouri  clay  being  of  finer  texture 
than  any  of  the  others  fits  in  well  at  the  end  of  the  groups.  How- 
ever, the  variability  of  texture  possible  in  any  locality  makes 
it  difficult  to  do  more  than  suggest  that  the  differences  in  shrink- 
age in  the  flint  and  semi-flint  clays  depend,  first,  upon  the  amount 
of  colloidal  material  present,  and,  second,  upon  the  size  of  grain 
and  texture  of  the  crystalline  parts. 

Knote  found  also  that  changes  in  specific  gravity  of  flint 
clays  upon  heating  differed  from  those  in  plastic  fire  clays,  the 
semi-flint  clays  showing  intermediate  variations.  He  further 
found  that  kaolins  behave  in  this  regard  very  much  as  do  the 
flint  clays,  a  fact  which  accords  well  with  our  finding  that  crystal- 
line kaolinite  is  the  principal  constituent  of  these  clays. 

The  "soft"  and  "plastic"  clays  derived  from  the  flint  clays 
should  show  varying  degrees  of  difference  in  their  physical  changes 
upon  heating,  depending  mainly  on  the  extent  to  which  meta- 
morphism  has  acted  in  dehydrating  the  kaolinite,  and  weather- 
ing in  breaking  down  the  semi-plastic  clays. 

The  porosity  curves  given  by  Knote  (/.  c.)  show  a  lower 
porosity  at  all  temperatures  for  the  semi-plastic  clay  than  for 


60  "Pyrophysical  Behavior  of  Flint  Clays,"  Trans.  A.  C.  S.,  X,  p.  365-79. 

61  "Some  Chemical  and  Physical  Changes  in  Clays  Due  to  the  Influence  of  Heat,"  Trans. 
A.    C.    S.,    Vol.    X.     Knote   examined   several    of   the    flint   clays   under   the  microscope 
but  does  not  emphasize  their  crystalline  nature.      It  seems    probable  that  he  worked  with 
the  powders,  which  do  not  give  as  satisfactory  results  as  do  the  rock  sections. 


STUDIES  OF  FLINT  CLAYS  AND  THEIR  ASSOCIATES.          345 

the  flint.  This  may  be  explained  on  the  ground  that  the  former 
have  less  constitutional  water,  and  so  upon  dehydration  would 
show  less  porosity,  and  that  at  higher  temperatures  their  more 
micaceous  nature  would  tend  to  reduce  this  absorption  since 
Rieke62  has  found  that  muscovite  cones  become  denser  at  cone  10. 

It  would  seem  also  that  the  porous  structures  produced 
when  the  fragments  of  flint  clay  become  dehydrated  should 
make  the  material  more  refractory  except  in  cases  where  gases 
containing  fluxing  elements  come  in  contact  with  the  clay. 

Under  such  conditions  it  would  seem  that  a  less  porous 
clay  would  have  the  advantage. 

Those  who  are  better  acquainted  with  the  uses  of  the  flint 
clays  may  find  other  adaptations  for  the  data  obtained  in  this 
investigation. 

SUMMARY. 

From  geologic  occurrences,  dehydration  tests  and  micro- 
scopic examination  the  conclusions  are  drawn  that: 

(1)  The  flint  clays  have  been  formed  by  the  setting  and 
recrystallization  of  fine-grained  largely  colloidal  sediments  which 
have  been  purified  mainly  through  the  agency  of  carbon  dioxide 
in   the  waters   transporting  and   depositing  them,   and  by  the 
leaching  action  of  plant  roots.     The  products  of  recrystalliza- 
tion  are   mainly   kaolinite,    with   minor   amounts   of   hydrated 
micas. 

(2)  The    "semi- flint"    or    "soft"    clays    have    been   derived 
from  the  flint  clays  through  metamorphism  by  pressure  and  heat 
resulting  in   a  conversion  of  much  kaolinite  into  hydro-micas 
and  the  development  of  a  completely  microcrystalline  structure 
-of  coarser  texture  than  that  of  the  original  flint  clay. 

(3)  The   plastic   fire   clays   associated   with   the   flint   clays 
have  resulted  from  long  weathering  of  the  "semi-flint"  or  "soft" 
•clays  and  are  structurally  more  crystalline  than  the  fire  clays 
which  have  never  been  "set."     They  also  differ  from  those  clays 
in  containing  a  high  percentage  of  hydro-mica,  kaolinite  in  all 
probability  forming  the  base  of  other  plastic  fire  clays. 


"The  Effect  of  Muscovite  upon  Kaolin,"  Sprech.,  XLI,  p.  577-83. 


346          STUDIES  OF  FLINT  CLAYS  AND  THEIR  ASSOCIATES. 

(4)  The  change  from  muscovite  through  hydrated  or  hydro- 
micas  to  kaolinite  may  take  place  without  destruction  of  the 
original  structure,  indicating  the  possibility  of  an  isomorphous 
series  embracing  all  of  these  minerals. 

(5)  Knowledge  of  the  microstructure  and  composition  of  a. 
clay  may  be  used  to  explain  and  predict  peculiarities  in  its  physical 
and  chemical  behavior. 


